There's no set time to get used to doing these things. Take your time - people get hurt when they push themselves too far before they are ready.
As for the bumps, you have to keep a relaxed hold on the bike. Keep soft hands, dont grip too hard, and keep your elbows soft. When we're too stiff and rigid, it jars us even more and harder, thus scaring us even more. When your arms are bent and soft, they will absorb the bumps you go over.

