Give yourself an affordable budget for it and worry more about that budget than an overall bankroll - especially if you are not playing that often.
This^
You're putting yourself in an impossible situation if you're trying to build a live play roll from online micro stakes. Following good BRM rules, it would take a looong time to create a bankroll big enough to play live
AND continue to use good BRM.
So you can do as Debi suggests and not try to BRM your live play like you are building from zero to infinity. Or, you can carefully build a roll to the value of a single live tourney or two and blow it. Or, you can concentrate on trying to micro/freeroll your way to live events "directly". Or, if you are lucky enough to live near a live venue that accepts
partypoker Live Dollars, you can try to micro stake your way to a modest (or bigger) live roll and try it out there. By virtue of the fact that PPL$$ have an expiration date, and because live tourneys are usually much more expensive than online micro's, they kind of preclude a tortoise-like approach to building a roll.
I have played live with a budget. I have played play money until I'm the Donald Trump of play money. I have played online micro stakes until I've built up enough of a bankroll to increase stakes. But I have never tried to go from online micro to live event until the introduction of PPL$$ last year.