For me, this question arises in two ways.
I am employed with completely flexible, work-from-home hours, but I still have to work several hours a day. I need a way to note that on my schedule, but still take advantage of the flexibility of not having to work at 3:15 p.m. if the weather is perfect and my kids want to play in the park.
I also grocery shop about once a week. That takes at least two hours, so it needs to be noted on my schedule, but it can move around if a friend calls to have lunch together.
Maybe you are a student and have to study for three hours today. Perhaps you homeschool for five hours each day.
Whenever something is a time suck, it should go on your schedule! But while appointments go in at a certain time, the flexible time sucks should only be tentative scheduled, to give you an idea of how much you have to go and approximately when the best time to do it is.
I solve this problem by putting brackets around the untimed, flexible but necessary appointment (as in the green circle above).
Now I know at a glance what can be moved (if I must rearrange my schedule) and how many hours total are already booked.
(In the example, I ended up working from 8 a.m. until 3 p.m. and 4 p.m. until 5 p.m., during my kids' appointment. I moved my workout, which honestly should be in bracket, too, to 7:30 p.m.)
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