Now would be a good time! There's no reason for you to kill it by repotting, but who knows what's going to happen if you keep leaving it like that. Get someone to help you - someone who's done a lot of repotting (and don't be afraid to add a lot of grit to the soil to make sure it drains quickly. Is there a bonsai club nearby? Members are usually very good about helping with things like that.

ponytails can stay in the same pot for many years. if you want it to get larger you must feed it (fertilize) and water it well when the weather is warm and sunny. the roots are short and skinny, so don't get a huge pot and stick it in there. i've grown many from seed and haven't had any problem.
happy growing
greg

My mother had a large one for many years (it's in the ground now, in Orlando, FL). She always knew when it was time to repot it when it broke the pot. Yours will break the pot if you leave it there long enough.