I've seen teen/adult beginners do their first canter at 4-6 lessons (at 45 mins - 1 hour per private beginner lesson), and I've seen some kids who still aren't ready after months and months of riding because they just aren't together enough to do it safely.

When should you start cantering?

As they say, practice makes perfect! Once you're trotting around confidently in rising trot, it'll be time to learn canter. How long it takes for you to get to this step depends entirely upon your particular circumstances, but generally you should be cantering in under two months or so.

How long until you can canter?

Maybe about 4 months into riding I started cantering. I was riding with a trainer who let someone jump a crossrail (low ones) after 4 years of riding. Eventually I switched and I started cantering. It all depends on how well you feel about cantering and your riding ability.