In May, we put one cup out in the yard for the sun and rain to do their thing with it.  We put another one in the worm bin.  In August, both were intact. Indeed, pristine, apart from the yard and compost dirt that had started to cling to the cups.  No cracks, no clouding, no visible signs of breakdown.  You could have washed them and re-used them. We had to trash them, since the "Other" plastic designation didn't allow us to put them in the local recycling.

BYO cups this summer.