I dunno how u can't tell the "standard" by now... there's too many songs to not know it... but anyway, besian, don't confuse "bars" with "seconds"... I have no idea how you did that, but don't. But yea, 16 bars per verse is the standard.
However, if you pay attention, some particularly popular songs are moving away from the standard. T.I.'s "What you kno about that" only has 3 bars per verse, and "Bring em out" only has 3 bars per verse, along with only 1 bar for the hook. Lil Wayne's "Fireman" also only uses 1 bar for the hook.
Be careful though, some songs that move too far away from the standards tend to be harder to tell where the hook starts or ends.