Hey guys, we do have a wiki! http://www.dustloop.com/wiki/index.php?title=Movement_(GGXRD)#Roman_Cancel
The rundown:
YRC, 25% meter, do-able during startup/active frames AND opponent is not in hit/blockstun.
RRC, 50% meter, same conditions as YRC, but your opponent IS in hit/blockstuin; has least startup and most slowdown, 80% forced proration
PRC, 50% meter, recovery frames; the most startup (like, a lot!) and the least slowdown