Originally Posted by The Messiah
You play until you feel you don't have an edge in the game, and with a minimum starting BR of 20buyins, which is enough for nanostakes.
Pretty much this - if you somehow lose five buyins but you still have an edge over the players at the table (and you can still cover the buyins under responsible BRM) then in theory you should stay at the table.
Of course, if you've lost five buyins at the one table then you probably also need to ask yourself if you really
have an edge... but that's another issue.
Also to echo what's been said above you're much better off playing $2NL with a full buyin than you are playing $5NL with a short buyin.