In pag. 154 of the Campaign Operations book it is said that units can be repaired, salvaged and manteined with a "Repair" order given during a strategic turn. I think that's pretty clear, even if it is not said how many hours can be used. I suppose we are talking about 8 hours, because in the following dedicated chapter this is the usual duration of the cycle. But it is not specified.
To me the Chaos Campaign misses some of the narrative fun of running your own merc unit.
First, yes you need to accomplish the objectives to get paid . . .
. . . BUT you need to do it in a way that keeps you viable.
This means you cannot play the same way you do for a lance on lance Last Man Standing slugfest. You get a leg opened up, side torso open, or a gyro crit and now it is about protecting that mech to get it out of the AO. Did you go into battle with reloads for your ammo? Or do you need to salvage some enemy equipment to get the ammo type you are short on?
Second, you have parts & ammo but time is always the most valuable.
If you can get more of your force back into the field to strike before your opposition can get their stuff back in shape, you can force a battle that is in your advantage. I had a topic I posted years ago for a campaign, I spent hours (b/c I used FM Mercs [R]) to repair the moderately damaged, reload ammos, ignored minor armor damage, and then I had a chunk of hours left in my repair budget before the planned sortie for the next morning to crush the forces that fell back to their FOB after failing to break out. I had three mechs that could be repaired- use a salvaged UAC/5 off a Cataphract to replace a heavily damaged one on a heavy; rebuild the side torso of a Zeus 9S that had IS in single digits, or I think re-attach a leg or something on another heavy.
Third, salvage is too simplified . . . I am trying to kill that Supernova by blasting it's side torsos off to salvage the cERLLs in the arms.