I think it's not so much as an 'etiquette' question as it is the customer does not understand your pricing structure, and you have not firmly set your prices.
On Rover, sitters charge what they want, for whatever services they want. Rover does define a walk or drop in as a 30 minute time period. So, when pricing out drop ins/walks, you have to decide, how much am I going to charge for one 30-minute time period for 1/2/3 dogs, 1/2/3 cats, etc.
When you know what you want, you have to keep in mind potential customers don't know how Rover works, and may even think you an employee. Pricing can be confusing.
Decide what you want to charge, and explain it in your profile. For new requests always refer them back to read your profile for your rate structure/policies.
Lastly, it couldn't hurt to keep a close watch on the sitters you're competing with. I don't know going rates for sitters in your area, but you could be pricing yourself out of new business.
A drop-in is 30min regardless of the number of animals. If you charge for additional animals (as I think most people do) it does not increase the time.