How long does it take to build a model railway? 'Forever' you might say, since no matter how complete a model railway is, there is always something more that can be added or some modification just crying out to be made.
Let's revise the question. How long does it take until a model railway looks finished? Here the answer is more definite. A model railway looks finished once enough detail has been added that others cannot tell what areas of the model railway are complete and which are to have more detail added. The exact time required to get the model to this point depend on a number of factors and it is these that I intend to discuss in the rest of this article.
The most significant factor in determining how long it takes until your model railway looks finished is time. If you work on the model railway eight or twelve hours a day, every day, it will be finished much sooner than one which only sees a few hours work done every few months. This is obvious but it should be taken into account during the planning stage so that you start building a layout that you have some hope of eventually having appear finished.
The size of the model railway is also a significant factor, or is it? A larger model railway surely takes longer to build than a smaller one. Perhaps not. The size is certainly a factor when comparing the time required to build large or small model railways in the same scale but this is not necessarily true if comparing model railways in different scales. The smallest item that is readily visible is the same size regardless of which scale that you work in. This means that if you work in a smaller scale, items which were visible and would have to be included if modelling to a larger scale shrink toward invisibility in the smaller scales. You might consider it nice to have all of the rivets along the side of your prize locomotive that you just built in Z scale but if you make them big enough to be readily visible they will need to be greatly over scale and if they are to scale most people wont even notice that they are there. You don't need them for the model to 'look' finished. The same certainly cannot be said about the same locomotive in G scale where almost everyone will notice the missing rivets. In this case the rivets need to be there for the locomotive to look finished. The same is true of all aspects of your model railway. The larger the scale, the higher the level of detail that you need to include to make it look finished and the longer that it will take to build a model railway of the same physical size even though that size represents a much larger area in the smaller scale.
The locale of your model railway will also influence the amount of detail required to make the layout look finished. A country scene requires some rolling hills, a few fences, and a handful of trees and it looks finished. A town scene requires streets, lots of buildings, and even more people and cars before it even starts to approach the appearance of being finished. In a larger scale even more details need to be added. This is probably one of the main reasons why there have been far more model railways built showing country scenes than town scenes.
Choice of prototype is another factor that can influence the amount of time required to build your layout. One aspect of this is availability of parts. If you can purchase off the shelf parts that will fulfill (at least initially) most of your requirements, you can get your layout to where it looks complete much faster than if you have to build every part yourself from scratch. Sure you might want to build it all yourself in the long run but if you can buy something that will fill that hole in the scenery for the next ten years while you get around to researching the prototype building that you really want to go there then you wont have to look at the hole in the scenery in the meantime. The same applies to rolling stock. You might have a list of all of the locomotives that you want to run your model railway when you can find the time to build them but if there are suitable locomotives that don't look too out of place that you can buy to fill the gap in the loco roster until you can spare the time to build them then you will be operational much faster than if nothing is available that looks even remotely like it belongs.
Another aspect to choice of prototype is the actual level of detail that can be found in the real thing. Steam locomotives contain more visible details than diesels, ornate sommersault signals contain more details than colour light signals, a mainline junction station with train frequencies measured in minutes contains a lot more details that will need to be modelled than a sleepy country branch terminus that only sees one or two trains a day.
Having a plan and sticking to it will get the layout to the point where it looks finished much faster than if you keep changing your mind about the layout. If you are constantly lifting and relaying the track then your chances of actually getting the scenery sufficiently detailed to look finished is extremely remote. Of course unless you have already built a large number of model railways and know exactly what you want, you will probably lift and relay the track on numerous occasions as you learn the limitations of your chosen track plan and attempt to rectify it.
The choices that you make in building your model railway are of course your choices. You may decide to build a very large layout, in a large scale, of a complex junction, in the middle of a town, at the height of the steam era, of the most ornate railway company that ever existed. You might be extremely busy and only be able to spare a couple of hours every few months to work on the construction of this layout. This is your choice but in this case don't expect that you will get it to the point where it looks finished for a very long time (if ever). The important thing is that you plan your model railway adequately in the first place and one of the important aspects that needs to be planned as early as possible is the detail, not is the specifics of what details that you intend to have (this can wait until a bit later) but more in terms of what level of details that you will need to have for your layout to 'look' finished. Once your layout looks finished you can invite your friends around to see it or exhibit it at a model railway exhibition and only you will know how much more work that there really is to go until the layout is really finished.