[RPG] How many cantrips can I cast continuously out of combat

pathfinder-1espells

Playing Skull & Shackles and we got shipwrecked. My character has Mending as a cantrip. According to the books, cantrips are so inconsequential that they can be cast "without limit".

Mathematically, a character could cast Mending every 6 seconds healing a hole in a ship an average of 1200 HP in an hour. The question is – are there any rules that prevent a character from casting Mending 600 times in a row for an hour?

My DM didn't let me get away with it, which is fine. I'm just curious if there are any rules out there that might apply.

:Edit:
My purpose in asking the question is less about how to repair the ship and more about the casting limitations for a spellcaster. My DM and I agreed that between the two of us, we could find a way to make mending work. Our question was about whether or not my character could keep it up long enough for it to work. As someone pointed out, mending has a 10 minute casting time, so the math actually works out to an average of 12 HP in an hour, which would take several days to repair a hole in the hull of a ship. Several days of constant casting. At what point does a character become exhausted from casting cantrips? That is the question I intended to ask and it might apply to other situations in the future, beyond mending a ship.

Best Answer

The rules actually do cover exhaustion via constant spellcasting, but only in a specific situation: if you're taking a move + a standard action every round, or else taking a double move or full round action every round.

PFSRD:

Walk: A walk represents unhurried but purposeful movement (3 miles per hour for an unencumbered adult human). A character moving his speed one time in a single round, is walking when he or she moves.

Hustle: A hustle is a jog (about 6 miles per hour for an unencumbered human). A character moving his speed twice in a single round, or moving that speed in the same round that he or she performs a standard action or another move action, is hustling when he or she moves.

From this, it doesn't seem to pay attention to the distinction between a move action and a standard action. If you're taking a single, non-full-round action every round, you can keep that up for about 8 hours (elsewhere on the linked page, but not in the quoted text) in a day. If you're taking a move + standard or taking a full round action every round, you can keep that up for an hour instead. In either case, you can push yourself to continue by taking non-lethal damage (or just risking it, for single actions).

As a house rule, you might choose to count an hour of taking standard actions (casting cantrips) every round to be two hours of walking, so that "forced march" kicks in after four hours instead of eight, but there's precedent for allowing eight hours of standard action activity in the crafting rules, so I don't personally recommend it.

(I think, based on your edit and comments, this is the rules you're looking for with your question, not the answers that are more focused on the actual limitations of mending as a ship-repair spell. If I'm wrong, please let me know and I'll delete this answer.)