How does the attack dog killstreak work?
The dogs spawn for ~60 seconds continuously. There tends to only be 4-5 dogs active at once, but when one dies another replaces it by jumping into the map somewhere else.
However, the dog mechanic is quite a bit different in Black-ops then it was in World at War. Because dogs are harder to get (11 kill streak instead of 7) they now kill in 1 bite. But instead of immediately 'knowing' the location of enemies dogs are now attracted to gunfire, making them much easier to avoid
The attack dogs will continuously spawn for about 60 seconds.