There are several things to look at when considering replacing a drive or starter belt. If a belt is cracked, pitted or dry rotted then it should be replaced ASAP.
As far as normal wear, the belt should not measure less than 1/8 in under its factory measurement. If the belt is slightly worn and showing a glaze on the outer faces that ride against the sheaves of the clutch, but is still within 1/8 inch of the factory measurement, then you can take a fine grit sand paper and remove the glaze by lightly sanding the outer faces of the belt. This will increase the grip that the belt has on the clutch and will reduce belt slippage.