If you leave the app where you earned an XP Boost, your XP Boost Round timer will continue to count down. If you return back to the app before the timer runs out, you will be able to take advantage of the time left in the XP Boost Round.
As long as you are playing in the same app, the XP Boost will appear in each game you play until the XP Boost’s time is out!
If you are awarded an XP Boost when playing a game and you level up and are awarded another XP Boost, the time will be added onto your current XP Boost. For example if you are awarded a 5 minute XP Boost and then are awarded a 10 minute XP Boost, your XP Boost will now be for 15 minutes.