How can I get a fair charge for a 5-hour visit?

I have no idea where you are located and what might be the prevailing rates, but I suggest booking someone as a house sitter instead. It's a flat rate good for up to 24 hours of care.

However--and this is a big however--you need to communicate with your potential sitters about what this job actually entails since it is not the typical house-sitting request when the pet owner is most in need of someone to stay overnight. Under this scenario, the sitter may need to spend those 5 hours during the day. Some (or many, who knows?) of the sitters providing house sitting services may also have other jobs and need free time during the day for classes, their job or to service other dogs for walking, drop-ins, etc.

Just be aware of this and discuss with potential sitters. You should be able to find someone to sit your dogs for 5+ hours for a more reasonable price.