even the smallest of heaters if allowed to heat unimpeded until they reached thermal shutdown ( prob around 200*c ) will get there hence the reason for thermal fuses in the heaters.
In a heat load environment they will never get there with out some form of assistance. IE removal of the heat sink source IE water, or raising the temperature of the heat sink ambient temp to allow the heater to attain that level.
simply put in water, the heater might attain 20*f above ambient.
in open air with no heat sink, easily reach 200*f
but then how far and how fast is controlled by wattage
if you wish to see what the maximum temp a heater can attain under ANY condition outside of normal
simply buy one and disassemble it, what you are looking for is a device approx 1/16" inch in circumference, and approx 1/4" or so long, it is encased in a metal skin, it may have a colored tip, in the side of this "can" is printed a temp. That is a thermal fuse, and it "blows" to prevent the heater from over heating.
and yes if you take a heater out of the tank allow it to get "hot" then put it back in a tank of room temp. water, the glass tube will break.