Personally,I think 100 buy-ins for an MTT.At least 20 for regular STT's.I would go with at least 30 for turbo STT's.
These are all minimums.If I were playing regular STT's,I would like to have 20 buy-in+5 for my new level.If I got under 20 after playing my 5 games at the new level,I would go back down to the previous level.
For example:I am currently playing $5.50 SNG's.My BR is $220.I would wait until I have $275 before I move up to $11 SNG's.I will stab at 5 of them and see how I do,and so on.