How hot? One solution is P=EI, where power in watts is equal to the product
of voltage and current. So the voltage drop across the resistor multiplied by
the current in amps through the resistor will yield watts of heat created by the
resistor.
Now, if you know the composition of the resistor, you can look up the
specific heat of the material and calculate how much energy it takes to change
its temperature. You will need to know how many joules of energy have been
applied. Calculate the joules by multiplying the time that the power was
applied in seconds by the power in watts. Now look at the specific heat table
and it will tell you how many joules per Kelvin it takes to raise the
temperature.
If you have any physics experience, it will be very simple.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip