The bonus experience is awarded from levels 2 to 47. If you are level 1, you haven’t leveled yet. If you are 48 or higher, the bonus will no longer be awarded.

Example: If you gain a free level at 46, you will not gain another free level. This places you at 47. If you level again, to become eligible for the free level, you’ll be 48, and too high level to gain another free level.
 
You only gain as much experience as is needed to level from the level you currently are to the next level, at the time you receive the popup saying you’ve gotten a free level. If you level after you receive the notification that you’ve gained a free level, you won’t gain enough bonus experience for an entire level.