Fluorescent bulbs can go for years without replacement—even when the ends start to blacken. Don't worry about them until the light they put out starts to weaken.
Black ends don't require immediate replacement. But you should have new bulbs on hand so you can replace them promptly when they do burn out.
Bulbs with black ends can run for a long time and they won't damage your ballast at all. But you should have replacement bulbs on hand for the day when they begin flickering, take a long time to fire up or finally give up completely. That's because running a fluorescent fixture with a burned-out bulb will damage the ballast.