Pactio activation requires magical energy from the mage. In the very beginning you see Negi burns out when he has to cast while his initial four pactios are activated at the same time.
It's mentioned by Eva that Negi and Konoka are among those lucky to be born with naturally high magical reserves (which is something you cannot increase through training very easily).
I imagine pactios with less talented mages (remember that by the festival fight Negi is stronger than most ordinary mages) couldn't be sustained nearly as long.
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