| Necrostar |
I haven't seen a specific time listed anywhere, and I think that this is basically a GM call. There are three reasonable options as I see it.
-Undead receive a save every night at midnight. This is the easiest way to maintain control over an intelligent undead, as you can predict when the save will be made and any undead you may have control over will save at the same time. This is harder with incorporeal and spawning undead. You can confine the undead and re-control them if they manage to shake your control. I would never rule this way.
-Undead receive a new save every 24 hours. Functionally the same as above, but if you have a variety of undead you have to work to get them to save at the same time.
-Undead receive a new saving throw once a day, but whenever they damn well feel like it. They are intelligent after all. Is it going to try to buck your control when it's chained? when your expecting it? No. This is how I would rule. Necromancy isn't meant to be safe.
As for recommanding, I do think you could avoid the unpredictability of the save this way, but I don't think that is a good use of your turns; unless you just have way too many. Unless it has a good chance of breaking out on any given day, 25% or more, I wouldn't bother.
There are other options that you have as a necromancer to maintain control over your undead.
-Bestow curse. You can force your controlled undead to willingly fail the saving throw against this spell, and put a permanent -3 penalty to its will save for the purpose of saving against your control. A second curse could lower its wis score by 4 for the same purpose. That's a -5 to the save! no small potatoes. Depending on your DM, these values could be higher because of the rather specific nature of the curses. Go for a -4 and -6 respectively.
-Command undead is a lvl 2 Sorc/wiz spell, and stacks with the class feature. Again, you can force your undead to submit to spells willingly, so they don't get a will save against this. Plus it has a duration of 1 day per lvl. While it won't grant you the same level of control, it acts as a buffer. If your undead companion gets upidy and tries to escape your command in the clutch, at least it won't attack you or your allies.