Min. injector off time

[bruce plecan]

Please someone help me on this.
"They" say not to run the injectors over 85% PWM,
on time.  OK, from what I've read the lag time from
V on till injector open can range from .0012 sec 
Peak+Hold to .0018 sec for Saturated.  Right?
But when they say 85% if the injector was on for
85 days, and off for 15, then that would be a 85%
on time, but the injector might be crispy by then.
So what is a min. off time that would allow an
injector to "relax"+"cool-off" enough??..
  If there is this .0018 reaction time, would then 
a .002 sec. be considered a min. off time?????..
If not what would be????.
  Would be above be a new consideration for
a high rpm motor for injector sizing????...
  TIA  Bruce