For a certain application I will be using this [resistor][1].

The working conditions are:

 - Ambient temperature (max): 70 ºC
 - Power to be dissipated: 15 W
 - "On" time (max): 60 seconds

The selected power resistor is a 15 Ohm / 25 W TO-220.

The question is: is a heatsink necessary for a 60 second (max) operating time? Can I get away with a large enough copper plane? Assume that the down-time between operations is larger than the operating time.

If it was the case for continuos operation at 15W, I'm already aware of the need of a heatsink and how to calculate it, but I'm wondering how I should proceed for this "non-continuos" intermittent operation

  [1]: https://www.ohmite.com/assets/docs/res_tbh25.pdf?r=false