takes me about a month or two. and sometimes i am in the same boat as you. 
Too long.
Wait a minute!
You mean working on only one at a time is allowed?
[:D]
I’ve never really kept track of the time from opening the kit to placing it on a diorama, but I do tracj actual construction hours… Generally speaking, 20-30 hours for 1/35th armor, and 12-20 hours for 1/48 single-engine aircraft, one-three hours for a 1/35-2/32 scale figure, depending on how much repositioning…
I’ve got some kits I started 10-12 years ago, so if I use your parameters and count the time I let it sit, it could take twelve-fifteen years for me to finish some models… I have around 25 or 26 kits that are under constuction right now…
On top of that, all the kits I I build are for dioramas, so a kit is only part of the build… Just a sub-assembly, as it were… Might spend 15 hours on a tank, 2 hours on each of 3-5 figures, and 40-80 hours on landscape, terrain, foliage, structures (and their interiors)… Sometimes it takes 4-8 hours to make a tree… And if there’re three trees and all there little bush-buddies, well… You get the idea… Still, if I do my normal 8-hour day of building, I can knock it out in two weeks… But then I don’t go near the bench for a month, so I avoid 8-hours a day…
I’m retired, so I can spend as much time as I want with a kit… But even before that, it still was about the same, in hours…
And don’t let that “detailed out the Wazoo” thing fool ya… Many of those “pain-staking accurate and detailed” models are done with one or two pieces that were bought after-market and painted in an hour or so, then installed… For instance, I used a resin cockpit for a P-39 once (it was a gift from the “Secret Santa” group build, I didn’t buy it, so no “Ah-HAA!s” from any of you clowns) that was one-piece plus a control stick, and it was painted and installed in less than an hour… The kit’s cockpir woulda taken about three… But, I’m “old school” and LIKE to make my own detail parts… That way, there’s not another kit exactly like it out there anywhere… It’s all mine, lol…
Old-school modelers like me spend time on kits… The others just spend money…
But all-in-all, it ain’t about how long it takes, its about how much enjoyment you get out of it… The faster you go, the less you get…
A few months but I build slow and only when my wife is working nights or weekends.
I’d say that I average a build in 1 to 2 months. It depends how excited I am for the kit, how much detail is in the kit, and how complex it is.
I just finished a Hasegawa F/A-18 that I was really excited about. That one took roughly 6 weeks. I also have a Polar Lights USS Enterprise that I’ve been working on for 2.5 years! Heck, I have an Airfix 747-200 that took me 16 years to finish.
I imagine that most guys in here are like me…I have a hard time settling. What I mean is that if I see an error in a build I have this urge to correct it…even if I’m the only one who is going to notice it. If the paint didn’t turn out exactly the way I wanted it I’ll repaint it or touch it up. If I notice a hairline seam after priming or even after painting I’ll fix it. If the model isn’t sitting perfectly level I’ll bust off the landing gear and make it right. This is especially true on an expensive kit.
Plus, I spend alot of time looking through photos of the subject on the internet so I can build it accurately.
Another good reason to build dioramas… If that happens to me, the plane gets a tarp thrown over it there, with a figure pulling it into place, lol…