Some challenges might have a small lock icon (similar to the Challenge by Status) with a Key:

The time in which the lock is activated changes between types of challenges:
it can appear once the challenge opens or even the last few hours.

Currently, All challenges lock in the last 24 hours apart from "Mega Prizes" Challenges which are locked from the start

If a challenge will lock in a time that is different than the last 24 hours, it will be stated in the challenge details.