I've always heard 24/7 for the refuge if you want it to help with PH swings at night.
However I've also heard some say 24/7 isn't necessary with Chaeto because of the low risk of it going sexual so they run it the same period as the DT. I would think they're missing out on the PH benefit though.
Anyway I currently run my refuge with the same photoperiod of the DT but when I receive my new HOB fuge this week I'm going to switch to 24/7.
I'm sure there's more experienced people here that can be more helpful though.