do you mean a specific time (e.g. from 8:00 AM to 05:00 PM) or Time Quota like -> the user is allowed to be online for about 5 hours?
There are some properties you can use.
Take a look at the mwg help directly in the GUI. Ich you take a look at the List of properties there are some more things possible. :-)
Hour (in 24-hours format, for example, 1 p. m. is 13)