What is the MakerBot Grip Surface lifespan?
There's no quantitative formula for the lifespan of any grip material, be it tape, glue, or PEI sheet.  If you find a lack of stiction and it's not due to small changes in the Z-origin (50 microns can make a big difference in the way the first layer grabs the bed!), then replace the tape.  It's cheap.

Just to be pedantic, torn and scarred tape should be replaced :-)