Well, as a professional singer taking vocal lessons, recording, having a photo shoot, etc.....It's the same idea. In the professional singing world, most of this stuff costs money. So, time is utterly important. If you pay for two hours of time, that's what you'll get. The person you're working with has organized their schedule just for you. If you try and get more time without paying, you are no longer viewed as a professional singer.
Word of mouth travels fast, and the industry isn't as small as we think it is. So, you will be known as the professional singer that wants to get more than what they pay for. Who wants to be known as a professional con artist? Good luck on that if you want gigs or a career.....
So, if you agree on a start time and end time, do what's expected. This is your career. If you don't take it seriously, no one else will.
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