Is there a cool down for activating steam keys?

Simply: yes.

I don't know details.

If they are all legit then there should not be a cooldown

30 minute cooldown after 3 failed attempts, I think.

you probably entered a few keys incorrectly before...you get locked for like 30 min...i'm not sure about how long

If you attempt to activate keys that have already been used, you get locked out after a small number of attempts.