Depends on what it's made of and how well it's cooled. We use 0603
surface-mount resistors at half a watt, because we heat sink them
well. Half-watt carbon resistors will die at half a watt in a confined
space.

Enameled wirewounds are very tough, up until the enamel melts.

Some resistors will die from temperture cycling stress.

But in general it's good to derate 0.5 maybe.

There is another dimension to consider: duty cycle. I've been told
(but don't have first hand experience) that old fashioned carbon
composition resistors are much better at pulse loads that newer
carbon film resistors.

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These are my opinions, not necessarily my employer's. I hate spam.