On the first set of Bridgestones I managed about 10,000 miles before both needed changing. They have a wear indicator (little ridge) that is spaced around the centre groove of the tyre. When your tread is the same depth as this, it is best to change the tyre. Likewise the brake pads, these also have a wear indicator on them, look down from the top (I think) and you should see what I mean.
Good luck
Nobby
