It depends on the traveler. My pace might be too much for another - or seem slow to someone else. If I were booking tours, I'd leave open half my time for other things that might be interesting. Even if you take a tour, it rarely has to be booked far in advance.
IMO the best place to pace yourself is on travel days. Those days where you wake up in one location and go to sleep in another. I've found a day that includes packing, checking out, eating breakfast, getting to the train station (or bus station, airport, or waiting for a taxi), making your connections, getting lunch, finding your lodging, checking in, unpacking and finding dinner is already a full day. I have a guideline that I'll plan only four hours of travel, sometimes with a mid-day stop at a sight, on a particular travel day. That keeps me sane and relaxed.
I don't believe I have ever come home from a trip having seen the exact sights, in the exact order, that I planned at home. I'll normally see at least a few unplanned sights and often skip something I had planned, for one reason or another. For that matter, I often pencil in, say, three nights in a particular location but end up staying an extra night (or two) or check out and move on early - depending on how it's going. It's important to me to never be a slave to an itinerary.
On a touring day (wake up and sleep in the same lodging), I plan roughly two sights a day, each day of a trip, no matter how long the trip. My normal pace is breakfast, one sight in the morning, lunch, a second sight in the afternoon, a break, then dinner. I consider that a reasonable pace. I'm fairly energetic, so I tend to walk more than the average person. I don't love guided tours because they're paced for the slowest people in the group. I also like to choose the sights I want to see, so longer tours are generally out. My most chill days are laundry (half) days. Breakfast and lunch are usually eaten quickly. Dinners are purposely relaxed and lengthy. I'm not really interested in active nightlife, but I will see a concert (jazz, opera, classical, traditional, dance) occasionally, never as part of a tour group.
I rarely book anything in advance, so I'm flexible. Some days, I decide I'm not really interested in the sight I had planned. Other days, I might have planned an outdoor activity, but poor weather (hot, cold, wet) makes it less appealing. Still other days, unplanned closures change my itinerary. Will I still see the sight? Maybe, it depends how I feel the next day? Sometimes a sight planned from home seems less appealing when I'm on the road. I keep a healthy list of alternative sights. If I find myself with extra time and/or energy, I'll look through those for something appealing.
It's not just what you can handle, it's also what those you are traveling with can handle. Sometimes I drop sights because those I'm traveling with need a break. I'm a very happy solo traveler but also enjoy traveling with others as long as they are morning people (ready to go early enough to beat the crowds). I've traveled with my mom. She does great until about 5 pm. At that point, I have to load her into a taxi, get her dinner then she's ready for bed - but ready to go again in the morning.
