Hi Steve,
How long the rubbers will last depends on many things:
-What brand your rubber is
-whether you use a Bucket-on-a-belt
-whether you use hard or soft rubbers
-your technique.
Speaking from experience, we use Unger Soft rubbers, and what I have found is that after about a month you get a small nick in them where they lean on the bucket on a belt.
This nick causes a snail trail, and as soon as that happens we replace them.
This usually happens long before the ends get worn, but this is the next point at which you'll need to replace them.
Phil