I have a regular client who prefers 1 hr. visits. As I have recently transitioned to being a full-time sitter, I had several things to consider in determining how to charge fairly. Not all of these considerations may apply to you, but hopefully, this will help.
First, I didn't think it was necessarily fair to charge her the full price for 2 half-hour visits, as I factor in my travel time in setting prices.

Second, I consider the client's expectations for the stay. In this client's case, she asks that I feed her puppy, take him out a few times on leash in their fenced back yard, and play with him in a manner of my choosing for the remainder of the visit. The overall effort that this requires, comparatively, is very small, so I considered that in setting my hourly price as well. Third, I know that the client wants one consistent sitter, and as the puppy was only 10 weeks old when I first started caring for him, I knew that if I could set a rate that the client was happy with, I could potentially have regular business for many years to come. This means that I had to consider my rate in comparison to other sitters in my area, as well as my skill set. I have a good deal of experience in veterinary medicine, so clients might be willing to pay a little more for my time, but there are also a lot of other seasoned sitters in my area, so I have to remain at least a little competitive in terms of pricing. In the end, I decided to charge my base rate plus 20% for the additional half hour. I feel adequately compensated, and I have gotten the benefit of watching my new puppy friend learn and grow, which is obviously compensation in and of itself. I hope this helps you set a price that works for you!