For cash games, I would recommend a much higher amount than 20 buyins. You can very easily lose 3-4 buyins in a session and to lose 20% of your bankroll in one session isn't wise at all.
Here's a link I found to bankroll management guidelines. Check out the bottom where they list Pro vs. Protected vs. amateur who will likely have to redeposit.
http://www.pokercurious.com/questions/view/what-are-proper-bankroll-management-guidelines/
