One week. I'd do water changes on all the tanks before leaving.
I have one very underweight betta who I'm feeding high-protein meals to 4x a day, so I would buy an automatic feeder for him. All the others should be fine without food for a week.
I have really lousy friends and nobody else I know seems to care about fish, so I wouldn't ask anyone to feed them or anything. If I did, I bet they would overfeed and cause constipation.
Fasting for a week is good for healthy humans and fish alike. I do week-long fasts myself bi-annually.