IIRC, Homura's time stop goes on as long as she wants it to. She's arguably the most powerful character in her series because of that, except that she mostly uses human weapons, so characters who have great armor would be unbeatable (unless they have a handy weapon she can steal or a spell she can throw them in the way of or something).
I heard it went like this for the series
Madoka: Most powerful
Homura: Most skilled.
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