LM317 / linear regulator, estimate of heat capacity
Over just 100 ms I don't think the thermal capacity of the packaged part matters as much as the thermal capacity of the chip itself. The regulator die may reach a very high temperature, and fail, before the heat can be dissipated to the surrounding package.
You also need to worry about the maximum instantaneous current. If you are passing a &quot;high current&quot; then the average temperature of the part may be the least of your concerns.