The truth is, everyone learns at their own rate. Some people are up and cantering a whole course in no time, and for others, they are trotting courses for at least a year before attempting to canter. It all depends. Some great exercises to build strength include riding without stirrups. This is brutal but it really does help...even if you can only go one time around the ring. Other great things would be trotting and cantering in two-point.