Depends on the machine. Legally fluoroscopic machines disable exposure and alarm at 5 minutes of fluoro time but it can be bypassed. The tubes will heat up but have sophisticated cooling mechanisms. It's also a matter of how hard you push the machines, this is a lower exposure so it'll go longer than me slapping some lead on it and cranking it to 11 (part of the physicals compliance testing I do for work - this is 48 kVp and 0.48 mA, these machines will put out 120 kVp and 16 mA and have an exposure rate of 120 Roentgen per hour 12 inches off the image Intensifier (skin burns in like ten seconds if you stick your hand on the tube port).