When do you replace a blade?

First, check to make sure the blade is not missing teeth or worn out. If so, replace it.
Second, if the blade is still good but not cutting it means it is
glazed over. Run the blade through an abrasive material (i.e. asphalt,
masonry block, sandstone or dressing stick); this will expose the
diamonds underneath. If the blade glazes over again during use, the bond
of the blade is too hard for the material being cut. Simply dress the
blade again by passing the blade through the abrasive material to expose
more diamonds.