how long can you keep yard mulch?

Answer 1

Well, until it decomposes and turns into soil. Basically as long as you want.

Answer 2

Yours is a good question. Rule of thumb is that mulch should be maintained at a three inch depth to be most effective. That means that most mulches must be added to at least twice a year in temperate climates (where there is no snow) and at least once a year in colder climates.

If you are using the new synthetic mulch, you buy it and don't have to replenish for a couple of years--that is because it sinks down into the ground unless there is an underlayment weed cloth below.