I don't personally recognize a "time limit" as such on the construction of model kits.
I've had Revell's 1/32 MiG-21 on the go, on and off, for the past four years, not because it isn't interesting and I'm getting bored with it; I'm still enthusiastic about it. That particular kit simply has some shortcomings that take some thought to properly rectify.
I don't think there can really be an average time frame for the completion of a model, because there's no such thing as an average model kit. Two different companies could produce kits of exactly the same thing, but theres no rule that says the kits have to be engineered the same way, or that their parts breakdown must fit a standard. The way a company breaks down its parts in a kit will often determine the ease or difficulty, and consequently the time involved in building it.
The time the kit was produced will also determine the time it takes to get it together right as molding technologies change and get better over time.
Case in point:
In the mid 1980s, I built Monogram's 1/72 EF-111 Raven. It was a good kit for the time, the parts breakdown made sense and I had it completed in about a week
and a half
In the early 1990s, I built AMT's 1/72 EF-111. It also was a good kit for the time. Its parts breakdown was a bit more detailed than the Monogram kit, so the increased number of parts led to a slightly longer building time, about two weeks total.
Two years ago, I found an old Airfix 1/72 F-111E on consignment at a local hobby shop and figured it would be an interesting challenge to build a kit with engineering from further back in time. What a challenge it was. It had fewer parts than either the later Monogram or AMT F-111s I had built, and about three months after starting it, and toiling over poor fit and rectifying shortcomings that I could do something about; I gave up.
There you go, Snoopy. Thats the sort of experience that my viewpoint on the matter comes from.
A lot, of course, also comes down to the modeler and what he or she has the attention span for. I can tell you this, if your'e an impatient sort who likes things to go easily without a lot of time involved, do your research before buying your kits, if theres a kit your looking at and its a fairly new release, there's a chance that some hobby magazine has written up a review of it, so you can get an idea of what you're getting into without really commiting to it.
For all those impatient modelers out there, if you're ever in a hobby shop and your'e fortunate enough to see Italeri's old 1/72 C-119 Flying boxcar, its not for you. Its a beautiful kit, even by todays standards and I've built it twice (took me the better part of a year both times) Its quite a complex kit, but well worth seeing through to completion if you start it. Just know that there are no short cuts with it, you're in it for the long haul with that kit. :-)