Use the standard T=5RC formula - use this to calculate the power rating of the resistor (P=I*I*R)
Standard PSU circuits use about 3 to 5 minutes to discharge to a safe level.
Anything above 40V DC is considered dangerous for human consumption.
Other things to think about...
Discharging a Cap too fast is like a battery short circuit = it will 'explode' in your face.
There's a lot of energy in capacitors... be careful, take your time.
