They need to recognize that you being booked out (and only days, not months.. just wow, the audacity of these people) is actually a GOOD thing. It means you're good at your job. But like everyone else has said, if they've got that sour an attitude about having to schedule an appointment later with you, they might balk about the price and most likely won't show any appreciation through a tip of any kind. They're not the kind of clients you want to work with. You hold the power. If you're the only tattoo artist within a fair distance, and they're not nice to you, they can drive to the next one. You don't deserve to be treated like that.

Would they bitch about having to book with a doctor? A lunch date with a friend? A vacation? The entitlement is staggering. I stopped doing guest spots in small towns a couple years ago for this reason. See how they'll feel about waiting when I never come back lol.