Apologies, I got to the zeroes and stopped looking!
The most we've done in a single day is three - Monastery of Alcobaca, Batalha Monastery and the Convent of Christ in Tomar, all in central Portugal. They're all fairly close together and obviously similarly themed, so I combined them into one video as well. It was a long day (we left Lisbon around 8:30am and arrived in Coimbra around 6pm), but I felt like we gave each site enough time.
We've done two sites in a day a few times, but usually only for either logistical reasons, or if one or both sites are quite small. Eg while in Cologne last month, we visited the castles of Augustusburg and Falkenlust in the morning, then spent the afternoon at the Cathedral. Natural sites and large cities usually take the longest to visit - we took two days to do the mines of Cornwall, and two days to do the Jurassic Coast in Devon & Dorset. And larger cities like Budapest, Vienna, etc usually require more than a day's filming, though we often save time by skipping on museums as we've often visited those cities before.
In terms of breaks, we usually try and take a full week off at least every couple of months. We had a period in July/August where we spent 10 days in Scotland, 10 days in Ireland (both NI and RoI), then a week in Manchester visiting relatives and a week housesitting in London where we only visited three WHS. That was probably our longest period of downtime.
My wife does 90% of the planning, as she's amazing at it. For 2017 we didn't have much of an overall plan, but we'd plan each day about 2-4 weeks in advance. For 2018 we've got a basic week-by-week plan for almost the entire year, though we haven't booked anything past about January 20th yet. Preparation is usually a mix of wikipedia, reviews on here, UNESCO summaries, official tourism board websites and other blogs we come across. I like to have an idea of what we're walking into, but at the same time to still be surprised! And yes, it's usually just revising the night before or the morning of - more than that and I'll start muddling the details!
Producing the videos is actually the easiest part, really! I'm working to a fairly set template, so it only takes an hour to edit together. It helps that I've done ~175 of them now, so when we're at the site I know exactly what footage I need. Today we went to Ravenna - picked up a hire car from Bologna airport at 10, drove to Imola and paid respects at the Ayrton Senna memorial, then drove to Ravenna. Thanks to preparation, we knew where to go, where to park, what we would find in each basilica. Including lunch, the whole visit took 4 hours and I didn't feel like we rushed (though admittedly we only visited 6 of the 8 monuments). Drove to our Airbnb about an hour south, which we'll use as a base for visiting San Marino tomorrow and Urbino the day after. Then back to Bologna the following day, drop off the car and get a train up to Milan for the Last Supper, then onwards to Turin!
Life admin eg haircuts, doctors visits etc usually happens when we have half-days. We mostly stay in Airbnbs so we often stay in and cook (though this varies by country and city).