Ummmm, ...
Depends what you are talking about.
The Bell 206, Hughes 500, S76 and many others have "auto relight". Some operate on Q others on N1.
If the donk quits with these systems the relight is almost immediate (assuming you have fuel and the engine is mechanically OK). Gauge needle jump and tail wag are about all you will see.
If you are describing an engine at zero N1, then yeah, it's gonna take a while to get her goin'. I have relit a a C250 in 1000 feet.
Try this, time how long it takes to get your engine to flight RPM - say one minute. Then calculate your auto descent rate - say 1500 fpm - and there's your answer.
Some old guys (not me) when flying a 205 would pull the starter trigger as soon as the engine quit - not to engage the starter so much but to turn on the igniters.
Last edited by oleary; 27th Dec 2015 at 05:31.