How long are DTC codes stored for?

I would have thought this has been asked before (and I bet it has), but I apparently don't have the right mad search skillz because I couldn't find it , so I'll ask afresh...

How long (in terms of time or startups, whichever is relevant) are DTC codes stored in the engine computer? Last night my car threw a code that I want to make sure the dealer sees (long story) and I don't want to accidentally "lose" it before I can bring the car in.

Thanks!