Based on my experience I'd say two months is near the minimum time, assuming that you're executing trades manually. I was able to monitor my game a lot while doing my stock run, 12-18 hours on workdays and 6-12 on weekends. I wasn't perfect by any stretch of the imagination, but if you can only put in half as many hours per day it would probably take you close to 4 months.
If you're knowledgeable enough to set up tools to do trades for you automatically, I think you could get to the finish line in 5-6 weeks. I'm sure it's possible, though I can't really offer any advice on doing it.
The fastest method for most people is probably what Sharon did: set up a macro to automatically hire girls, level them, sell the company, and repeat. That will tie up your computer while the macro is running, but if you can let it grind for 18+ hours a day it takes about as long to max your decos as playing the hell out of the stock market manually.
I agree that the time it takes to upgrade all decos is NOT worth it, from a tournament perspective. You'll barely notice the difference in tournament between having all decos around level 50 and having all of them maxxed. That's exactly what I did, and it gained me about 500 girl levels. I was unimpressed.
Deco upgrades are almost pointless past level 50.
The shop multiplier is the #1 factor determining how high you get in tournaments, by far. My advice is to focus on that if you want to advance.