Anybody’s/anyone’s guess
Definition 1 of 1
Pro Tip 1/3Uncertainty Expression
Use it to express that the outcome of a situation cannot be predicted by anyone. 
Whether we'll have good weather on our trip is anybody's guess.
Pro Tip 2/3Equal Ignorance
Implies that no one has special knowledge about the subject in question. 
Who she'll choose as her dance partner is anybody's guess.
Pro Tip 3/3Casual Context
Best used in informal conversations rather than formal or academic writing. 
How long the meeting will last is anybody's guess.