General rule I go by is a little warmer than room temp, ie it should feel warm to touch but not at all hot (although I have come accross alot of conflicting advice), if it is still a little too hot your beardie will not like it and let you know (they will thrash about to try and get out) put them in tail first after you have tested the water.
The main issue as long as you hand test the temp is keeping it warm enough, if the temp drops too low your beardie can go dormant and therefore you may just think they are comfortable. so keep testing the water every 5 mins and if it feels colder than room temp add some warm water, but not too close to your lizard.
Mine will happily
bath for 20 or 30 mins, I
bath them in my sink and once they have had enough they will hop upto the side. I try to
bath them every 4 days.
If using your sink, clean your sink with reptile safe detergent beforehand, chemicals in soap residue can be harmful.