it all depends on what you plan on keeping in it, assuming you are going to keep macrealgae in there, then you have 2 options:
1). light kept on 24hours a day, 7 days a week! This is supposed to be good when keeping caulerpa (personally I would recommend cheato) as it can stop the macroalgae going a sexual and becoming a nuisance!
2). light kept in reverse with tank ligting ie. refugium light comes on when tank ligting goes off and goes off when tank lighting comes back on in the morning!
The second option is recommended by most as it apparantly helps stablaise pH levels and keeps the tank in balance and it also a bit nore natural to help macroalgae grow!