You might have seen a more complicated variant of this question going on about different types of speed but a comment by Szega made me realize that a much simpler question would probably suffice and that if ever another situation came up where I needed that more complicated answer I could simply ask a new question.
If a familiar spots a hidden enemy, but the corresponding wizard does not, how much time does a familiar need to communicate that danger to the wizard?
RulesI predict to be relevant:
While your familiar is within 100 feet of you, you can communicate with it telepathically.
A member of a group can be surprised even if the other members aren’t.