A stretch certainly is a lot easier to manage than a shift! Although I have the same question about vehicles now
The answer to my other question, then, is that since you're allowed a stretch or two of leeway anyway when doing a shift activity, if you repair your guns then you will also probably maintain them in the same shift. Doesn't require any additional thought or ruling really. For vehicles, I guess we'll see.
In play this whole thing is a little more complex. Do you set a timer for each week and people can try multiple times per week to successfully do their maintenance? I hate that. Do you do the check once, whenever each character gets around to it, and the results of that roll stand for the week? I don't hate that as much, but it sure does add to the pile of recordkeeping that you have to do. It also gives very different results in terms of rate of decay, but maybe the intent is just that you'll need to find more time for repair rolls.