Opening & closing times vary. Most amps open around 10 AM and close around 3 or 4 AM. However, a few close at midnight. For example, Oceanside tended to close at midnight, as well as The One (and all its other names). Some of the amps on the Emerald Isle seem to remain open 24/7.
Some of the girls have mentioned having customers arrive as early as 8 am and it is likely someone will answer the door if you knock that early. However since they probably didn't get to bed until 3 or 4 am, you may be meeting a very sleepy headed companion that early.
Have fun and play safe.
SL