You can add the cleanup crew pretty much as soon as the cycle is over.
You should be testing either daily or every other day for the first few weeks.
You should first see the ammonia test spike, then the nitrite test should spike, and once the nitrates start coming up (and the others read '0' mg/L) you can do a 20% water change and then add the cleanup crew.
I'd start out with not too large of a crew, and add more as you need them... there really won't be too much for them to eat until the algae's really start blooming.
I started off my crew with 10 hermits and 10 Astrea snails.
My cleanup crew now consists of 16 hermits, 8 Astreas, 2 Mexican Turbo snails, 1 Turbo snail, 1 cerith snail, 9 Nassarius snails
, 2 Queen Conch's, 2 Fromia stars, 2 tiny brittle stars
(hitchhikers), a 1/2" crab (another hitchhiker), and an Algae Blenny.