I think now would be a good time to add it while its cycling, other wise, you have to add fully cured LR
at a later date, which a.) costs more money, or b.) is a bit of a hassle to do your own unless you're up for it.
I would add all the rock you want right away. Just watch your water for its quality. You need a skimmer going if you add lots of LR
, and may need to do PWC
while it goes through the die off part.
The more water you change, the slower your cycle will go, but the more life you'll save on the LR
from dying in an ammonia cesspool.
Others will certainly advise you better on this than i can though.