in case of normal power failure, are egress/emergency lighting fixtures required to turn on instantaneously, within 10 sec, when genset turns on, or other?
7.9.1.3 Where maintenance of illumination depends on changing from one energy source to another, a delay of not more than 10 seconds shall be permitted.
Agreed. The generator is required to be on-line within 10 seconds, if there are "emergency loads," such as egress lights, that it will have to support. So the lights themselves can be off for that same 10 seconds. The building occupants will just have to put up with the darkness for that short amount of time.