Heads up SNG's, bankroll question

[grouchie]

Recently i've begun playing more 6-max cash games and I have started playing some of the $5 heads up SNG's on stars.
I'd like to leave money in my Stars account for that purpose only and see if I can build a bankroll and continue to the higher games.

Only question is, with something that will have as much variance as heads up, what type of bankroll would one need?
is 50 buy-ins adequate?

anyone with experience i'm interested in hearing your advice.