How hot can MOSFETs get before blowing up?
Try to get the part number of the MOSFETs, and search the web, to get their datasheet. 

Power MOSFETs are often rated for operation upto 125C or 150C, i.e. they could boil water and still be within normal operating conditions. 

As Ignacio Vazquez-Abrams says cooler lasts longer. However, I would be surprised if a motherboard from a reputable manufacturer failed due to power MOSFETs first.