The strength will derate as temperature increases. In general, staying 100F below the temper temperature will maintain strength unless extended times are seen. Take a look at the Holloman-Jaffe parameter and equate the temperature/time used for the temper heat treatment to the same Holloman-Jaffe value for a reduced temperature of interest. This would give you an estimate of the time required at that reduced temperature to have the same effect as the temper heat treatment.
