There is no time limit for collecting the rewards you earn. You can claim the rewards anytime before the current Game Pass season ends.

If any reward from a Game Pass level can’t add to your account (e.g., Gem reward at 25,000 Gem balance or chests during inactive Collections season), they a...

Since level 100 is the maximum Game Pass level, if a player is on level 85, the Game Pass Skip 20 Levels will only be applied until they reach level 100.