The answer to this question depends on a lot
of variables. Sometimes they fly up well before
dark ( especially rainy days ) at other times
I've seen birds on the ground when it was
nearly dark. My personal preference is
to stay out of the area where they are roosting
until nearly dark when visibility starts to fade
and the chances of spooking birds is less.
Then get within a couple hundred yards
and try to get a gobble by owl hooting,
this frequently works but not always.
Good luck!