Normal thought is not to increase your training distance by more than 10% per week or training period, since that much more is usually OK for the body to endure, but it's not so much that you can't move the next day. That's primarily the technique for running, weights, etc, and I use it for bikes too, so if you can ride 20 with no problems, next time try 22, if that's OK then up it a little more.
While you probably could do 50 right off the bat if you went slow enough, if you have the time, give your body the chance to get used to it. If 20 is fine, then I'd try 25 or so, but if at 20 you're pretty tired, then give your body a chance to adapt. If not, you'll just injure yourself, have to take time off, then start all over again.