too many things to take into account to give an accurate answer to be honest. Firstly, you'll need to explain 'decent'... Not trying to be clever, just people aspirations vary drastically. If you want to play along to the radio, and strum a few chords you should be well at it in a matter of months. If you want to write your own stuff and shred like a freak you could be at it years.

I'd only been playing for 2 years when I got my first paying job. I see education as lifelong - I don't ever see myself stopping taking lessons. Maybe not with the same person, but I always feel like there's more to learn... and pretty much anyone can teach you something... The stuff on this site, and the wealth of others on the 'net is great, but I find I learn more quickly taking lessons - it also gives me the drive to practice hard each week, as I have a target to aim for. Teaching people also helps me learn, and I'd recommend that to everyone.

If you want to play like Satriani, be prepared to practice pretty much non-stop for a few years... and getting a (good!) teacher will definitely cut the time down...