Unfortunately this option is not available yet. I had a similar request a while ago with a bit of a twist.
I requested to have different transfer rates based on time of day. That way we could throttle the bandwidth used during busy times and then increase the bandwidth used for replication when the network was quiet.
What we ended up doing (which fits with your request) was to setup a cron job to start the replicator service at 6pm local time and then a second cron job stop it at 6am local time. You just have to be aware that newly captured images and snapins won’t appear on the remote FOG nodes until the next day using this process.
It would be nice if it was integrated into the GUI, and it might be in a future release. But for now the cron method does work.