XP Bonuses activate as soon as you claim them. If you aren’t prepared to take advantage of the bonus right away, you should hold off on opening a WCase or Package until you're ready.
You can have more than one XP bonus active at a time up to a maximum of 3X bonus. The duration is extended to the sum of all active bonuses, e.g.:
1.5x bonus for 3 minutes + 3x bonus for 2 minutes = 3x bonus for 5 minutes