The short answer I would give is about 20 months.
To take it a little further, your child should be able to pass three tests: bladder control, physical readiness, and instructional readiness. The bladder control test is: does your child have a good deal of urine at one time or dribbles through the day? If he's holding it and then releasing a good deal at once, he's probably ready. Physical readiness: can he pick up objects easily? Can he walk through a room unassisted? If yes, he's probably physically ready. Instructional readiness: can he follow simple directions, i.e. bring you a familiar object or sit down when told. If he can do that kind of thing, he's probably ready.
A great book to get would be "Toilet Training in a Day" by Azrin and Foxx. It was published in 1976 so you can probably hit a used book store and get a copy for a buck or so. Or look in your local library. I'm not trying to get you to spend money, but the book is a great help.
I knew Nate Azrin a while back and I have used this method on all three of my kids. It worked incredibly well with all of them. My little girl trained the quickest, but all three were self pottying by the end of the day. What a relief!! I have also trained two of my nephews and three family friends (the children). If you follow the program, it works.
Other than that, good luck and good training!