When it comes to flashing it is done my timing. Since most units don’t have temp control you let them come to full temp and time yourself under the flash.
The goal of a flash is to dry the ink in between colors. When your platens are cool this may take a little longer. I usually flash for about 10-12 seconds on cold boards. You just want the ink to be dry to the touch not cured. When you bring the shirt back in front of you, you can touch the image to make sure it is dry.
Now as you go along the boards will begin to retain heat and your flash times will decrease because you have heat coming from top and bottom. It takes less time to “dry” the ink. You can get down to 6-8 seconds on a flash then. The other variable to take into account is the height the flash dryer has from the garment. The closer it is the faster those flash time will be.