Phil, There are 2 answers to your question from my limited experience...

1) Try it until it melts.

2) Measure the EGT on a long high boost run to see it det/high temps start
to creep in over a period of time under high load.
(This is where engine dyno's are usefull)


On a recent 630 bhp engine done on my ecu, the mapper ran the engine
for 10 minutes on the dyno on full boost at peak torque to see how the EGT's and
other temperatures progressed.

Cyclinders/pistons/sparkplug/valves can all accumulate head under prolonged
boost conditions and although its ideal to tune an engine on the det limit,
often most tuners have a feel for what margin of safety is included in
the final map.

E.G DET=11 degrees, NO DET=10 degrees, SAFE=8 degrees (but slight power reduction)

The safety margin size depends on many factors beyond my knowledge !!