As several stated, life can vary greatly according to formula, storage temperature, dilution and air contact (or amount of air left in the container), not necessarily in that order.

Concentrated Rodinal can last years or even decades, a working solution of some developers barely longer than the developing time itself.

A trick is to keep the film leader and develop it in your developer for the normal time & temp: the emulsion side should be as dark as the back (or very dark, if you happen to have an especially light backing). At least that will show if the developer still has some life in it.