Here is the info right from UL I sniped out a bunch of other info and left in only the info you asked for.
From the 2004 UL White Book
CIRCUIT BREAKERS, MOLDED-CASE AND
CIRCUIT BREAKER ENCLOSURES (DIVQ)
This category covers circuit breakers and circuit breaker enclosures designed to provide service-entrance, feeder or branch circuit protection in accordance with ANSI/NFPA 70, ‘‘National Electrical Code’’ (NEC).
Unless otherwise marked, circuit breakers should not be loaded to exceed 80 percent of their current rating, where in normal operation the load will continue for three hours or more.
We can also look at the NEC in a few places here is one them.
210.20 Overcurrent Protection.
Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies with 210.20(A) through (D).
(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.
The short answer is you can load a breaker to it's rating for up to 2 hours and 59 minutes at a time. If the load will last longer than 3 hours you can only load the breaker to 80% of it's rating.
Feel free to ask more detailed questions.
Oh the outlets, in commercial you have to count them at 180 VA in residential there is no limit.
All of this is based on the NEC you may have local amendments.