If the floor with the radiant heating is under a wood, Vinyl or tile floor and the floor has good insulation the radiant heating should take at most 60 minutes to fully heat up and start giving off radiant heat.
If the floor covering is carpet then it may take a bit longer for the radiant heat to get through the carpet and start heating up.
My radiant floor heat starts giving out heat within 60 minutes of turning it on and my floor is wood flooring.