The rules don't explicitly state it, but it reads to me as 1 'charge' of a healer's kit/skill check/person (so long term care for 3 people would use 3 'charges' per day).
That said I can see where your GM is coming from, I can see an argument that looking after someone for longer requires more supplies than a quick patch job.
Ultimately your GM is always right. And a few gold isn't worth a 2 hour argument over. Now you know how he is ruling it you can ensure you have a few more kits on hand.
Today, we had a big argument at my table about the meaning of the supplies needed to provide long-term care with the heal skill our gm asked for 2 charges of healer's kit as you might imagine the one with the skill said forget it in a more colorful manner sparkling a 2hours argument . Is there any ruling about that issue?
No rules for this, so GM call... Personally I will, as dragonhunterq said, rule for 1 charge of healer's kit/day/person but I will also allow my players to use their clothes and go plant search for some light poultrice... So instead of a light activity it will be a full time activity if you don't have the material at hand.