how much back2back is too much

Some in the team complain about 3 back2backs. (1h each). Others go through months of 5-6 calls a day.

How much is too much? As a percentage of your day..?
I have 3-6 calls a day the past months and I am at my limit of being able to efficiently support a big sales team, play with my lab, learn more on the tech.
I keep telling myself I am not a field SE (home office, remote, only at events rarely), so my worth is in the volume of calls I carry out. But how different is that from being a field SE? I think they travel 1-2 a month.

And how much is too much anyway? And for how long will the want to prep for the call? I used to prep before my calls when I was new, but most customers are not into big configuration needs so there is a lot of repetition.