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:][1]

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.) [1]: http://www.d20pfsrd.com/alignment-description/movement

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.)