First, if you look at the actual mechanics of Homura's shield, it operates in such a way that it's more likely that she has a range in which she can stop time, not a limit on how long she can stop it for. The "mechanics" of the shield, insofar as they are not simply decorative (and come on, it's onscreen in PMMM), are such that the hourglass turns sideways and thus "stops" when she stops time. It's unlikely that keeping time stopped costs additional power.
Second, if you assume it does... that falls into the same trap as the weaponry itself. Where'd she get those grief seeds? Order of magnitude approximation: we see Witch numbers in the single digits across one month. Even assuming that for every one Witch we see defeated onscreen Homura defeats at least three, that still means that Homura can't get more than 15 Grief Seeds a loop. We don't know how efficient Homura is other than a WoG statement that she's the most efficient of all the puella, but assuming it takes three hours to mine the stadium, that's still twelve minutes a charge assuming she used them all at once. Which she is not likely to do - those 15 Grief Seeds are her budget for the entire loop, and I'm being generous even giving her that much. A more reasonable lower bound on her timestop is twenty to thirty minutes. Assuming there is a bound at all, which, again, I'm not convinced.
