24 hrs in dry temp controlled conditions...(per "specs")
Cold and damp will take longer..
I have siliconed in the morning...left it out on a picnic table on a sunny sumer day...and filled with water that night...
OK for baffles but not a "side" job or tank build...
Clear silicone starts out "white" and turns clear...
When it's "clear" it's cured...If it's "cloudy"..better wait it out...
go to Home Depot and get some MARINE GOOP , i used this glue on my sump , if you heat it up with a hair dryer it speeds up the process to us within 1 hour of glueing, no chemicals go into the tank. i've told tons of people about this and like it.