I've been playing since GT1, and have had DFP, DFGT, 2 Momo Wheels, a G25, and 2 G27's (3 ps1's, 2 ps2's, 2 ps3's so far).
I use a G27 currently, had it on a table while sitting in an office chair; that sucked.
Built a cockpit out of wood in college (roommate was a tech major, actually got credit for it). It was pretty sweet, worked well. A month of after-class play and I (and my roommates) were passably good wheel drivers (win races cleanly and consistently at the very least).
I now have a Playseat, big improvement in ergonomics and rigidity over my homemade one. 2 weeks or so with that and I can take just about any car around any track quickly.
So, if you ask me, it takes about a month to be good with a wheel from scratch, and a real cockpit/mount makes all the difference.