If you're interested then the best time to start moving towards anal is the same as the best time to plant a tree - today. Keep in mind that anal doesn't necessarily need to be with a partner, nor does it need to be penile-anal sex. In fact it really shouldn't be penile-anal sex until both partners are comfortable with anal stimulation and the roles they're going to be playing. You need to be able to trust the other person before you'll be able to relax enough to make it happen nicely.
Personally I would definitely wait for a long-term relationship, but that's just the way I am. I keep sex for my wife and we can trust each other 100%, and I don't think I'd be comfortable with it in a relationship that was any less.