I think it was about a month. I feel like the large variety of areas the kids traveled to had to take quite some time to trek across, but they also had to be swift enough to stop Giygas before he caused too much damage.
I think yolu could calculate this. Like, you make a full playthrough. Sum up all the hours playes and you get the idle time. Then you add a day for every time you use a hotel and that's it, you get how much time the story took canonically, but it will depend on your playthorugh