There is a rule of thumb, that an increase of 10 degrees C, halves the
life. To determine the mean time to failure, they generally run
accelerated life tests by testing at elevated temperatures.

That said, it may not make any significant difference to you. Back in
the 60's the better transistors had a mean time to failure of about
1500 years ( based obviously on accelerated life tests ). If your
triacs had a mean time to failure of 1500 years based on a junction
temperature of say 30 degrees C, then running them at a junction
temperature of 70 degrees might reduce the mtf to 90 years. Just
remember the temp limit is the junction temp, not the ambient temp.

Dan