Honestly, yeah... it's hard to say how long it takes to get comfortable with the canter because there are sooooo many degrees.
I'm comfortable cantering my personal horse... but I've been riding her for seven years, and her canter isn't particularly difficult. Even then, it took me about three years of consistently riding her (as a rusty adult re-rider) to get most of the unintended bounce out of my seat. Most of it. If I'm having a tense day, I can have difficulty sitting her canter, to this day. When I first started riding in my teens, I always rode canter in two point, so it was easier to adapt to differences in canter. These days I almost always sit the canter, but if anything doesn't quite line up for me on a given day, I can end up bouncing.
Some horses I can get on and feel fine cantering them during my first ride with them... and others feel like an out of control carnival ride. Every horse's canter is very different! It's normal to have some difficulty in the beginning of learning.... and the middle, haha... and then there's never an "end"....
