How long does beef stay fresh when marinating?

In a covered container in a cold fridge, covered in marinade, depending on the marinade anywhere from 3 days to over a week, as long as it is stirred.

It seems to be an oxymoron to call marinated beef 'fresh'. Depending upon the marinade, it could be adding acid, salt and/or sugar which could have some preservative properties that might allow for a longer shelf life compared to non-marinated beef.

Use a recipe from a trusted source and don't push the time table to it's limits.