Unless you purchased a hot clear plastic bag of thinset on the streetcorner, it should say on the bag. Or you can always go to the mfg website for instructions.
Here is the Versabond instructionshttp://www.custombuildingproducts.co...er=diy&lang=en
And yes, its 24 hours. Given a few little conditions:
If its been cold curing - will take longer.
Too much water - will take longer
Too thick -will take longer
In between two impervious assemblies - may take forever