I've been playing Toontown for a year now give or take a couple of months and that entire time I've only played 1 or 2 Toons.
Since I don't play alot during the day (because of work and overtime - family and such) and I DO play other games as well, I took 5 months off of Toontown to do some Beta Testing of other games with some friends its taken close to 9 months to max my Toon.
For most people it doesn't normally take that long but from personal experience I can tell you that its well worth having a maxed toon. I can solo the factory in CogHQ and upto a 3-4 story building in Donalds Dreamland.
Good for you for making that decision! If your like me you'll need to take a break every once and awhile and work on other things but the end result is nice.
Good luck to you!