If you can ride without stirrups you are ready to canter but... you first need to ride more often and you will find that sometimes the horse will happen to canter. Follow it a little bit and relax and then take a breath, relax and think "trot". I also think that if you can trot a few circles without stirrups you will learn not to rely on reins to ride but your seat. Nothing is uglier than someone start cantering and then panicking to grab the poor soul in their mouth to stop.
As soon as you don't use the reins and the horse's mouth to balance - meaning that you can keep your weight in your heels and trot without holding onto reins at all, posting and sitting, I would say you're ready.
My coach didn't let me until like a year and a half into lessons and that really set me back and she should have let me start earlier. Now I'm a barrel racer and I show hunter under saddle and huntseat equitation all over the state of Texas and I have a bigger disadvantage
It depends on how comfortable you feel.If you feel comfortable try sitting trot with no styrupps (I fell off the first time I done it but you just need to get backup again) when you feel comfortable with this you can start cantering
It doesn't have a X amount of time, it's when you are ready, some people take a few weeks some people take months it depends on you, your balance, your position, how strong your legs are if you are already confident while riding