Keeping the old information for historical preservation:
Original documentation from 2003:
Cocking a light crossbow has a 6 second RT and cocking heavy crossbow has a 9 second RT. The archer's strength bonus decreases this RT. For every +10 strength bonus, the RT for cocking your crossbow is decreased by one. The minimum RT to cock a crossbow does not go below three seconds for a light crossbow and 6 seconds for a heavy crossbow. Certain factors, such as encumbrance or being under-trained for your armor, can increase RT. The RT to fire a crossbow is 2 seconds flat.
In practice there is some minor variation in roundtime. With a strength bonus of +10, light crossbows can be cocked in 6-8 seconds and heavy crossbows in 9-11 seconds while standing and unencumbered. Cocking from non-standing positions takes 2 additional seconds.