Combat vehicles are EXTREMELY unreliable, even the most reliable ones. Roughly 10% of modern mechanized forces break down for every 100km traveled. As a former tank commander I can totally accept that. Every time we stopped we'd have to jump out, run around the vehicle checking the track, temperature, fluid levels, etc, etc.
For every combat scenario those rolls make total sense, as vehicles are pushed harder. When moving without combat those rolls make sense every week. When the vehicle is sitting in a motor pool not doing anything, it still needs to be started weekly. If you miss a few weeks of maintenance, there should definately be a roll.
According to the rules, each day provides 8 hours of productive work, but you can get 12 out of them (makes sense, mechanics in the field either work/sleep/eat). It doesn't matter how long your "turns" are, you get those 8-12 hours per day. This is the last paragraph on page 168 of SO.
Maintenance checks are between scenarios, and covered under maintenance on P169. This is in line with what I mentioned on how in combat situations, stuff breaks down, and breaks down often.
However SO doesn't mention maintenance checks over time when combat or mothballs aren't involved. I would recommend a check per week per vehicle if no combat, or per scenario is there is combat, even if there are 2+ scenarios in a day a roll between each scenario (but still only 8-12 hours a day to fix it).
I really think they should have provided a modifier for mobility types on vehicles though, wheeled vehicles require less than tracked, VTOLs more than tracked, etc as well. Maybe they thought this was included within tech ratings or something. I'd think Wheeled -2, Hover +1, Tracked +0, WiGE +2, VTOL +3, Fixed-Wing/conventional fighter +2, Naval -1, Submarine/Hydrofoil -1, Satellite -4, Airship +1, Rail -3, superheavy vehicles +1, superheavy mechs +1, etc. Hopefully something like this will be In IO or ATOW Companion, as there should really be a difference in maintaining a tech rated D "Apache" and "HMMWV", and perhaps even a time modifier. This was hinted at on page 172, but I think they tried to keep things simple or more likely just ran out of space, as the maintenance rules are pretty comprehensive.
I also think that the status tables on page 173-174 are a bit flawed, just a bit. For example if a vehicle fails maintenance by far the most common thing that is going to go wrong is the vehicle becomes "immobilized", or "deadlined" in military terms. The vehicle has some mechanical issue that simply prevents it from driving. I think I would replace the "1d6 armor damage" to "vehicle immobilized" like a critical instead, though its more catastrophic, it makes more sense logically. Reductions in speed are fine, stuff like "I'd keep it under 20, or you'll probably throw track or overheat". I'd also change weapon destroyed to weapon damaged, but with a 50/50 chance you simply don't know until you shoot it.... so, perhaps instead of weapon destroyed, it jams on a '2', adding +1 each time you get that same roll.
But yes, maintenance *IS* excessive, more so today than 100 years ago for sure. If the first gulf war would have lasted over a week, the US Army would have had a SERIOUS shortfall in spare parts.
Some interesting stats from James Dunnigan's "How to Make War" on this:
2% of combat aircraft lost each year in peacetime, in wartime its closer to 50%.
Rarely more than 90% of vehicles are in a running condition at any given time
2% to 20% of vehicles are lost per hour of movement from maintenance (long marches can easily hit 50% breakdowns)
Tank losses are 5-6x personnel losses in heavy combat, though 60% of those losses are non-combat and can be repaired
