Firstly, the perhaps obvious: what finding time for yourself looks like (including the need to do it yourself) does depend on the people, the dynamic, if you got to negotiate, etc.
Some issues you might run into include a) just having the time at all for your own things, b) managing that time (and the temptation to rest and relax when you catch a break, not engage in a high-energy hobby of your own), c) finding unbroken chunks of time where you’re not passing by a reminder of something you should and doing it, or having a timer go off for the laundry or starting dinner, or having, “Slave!” yelled across the house or whatnot. (There’s a reason I’m required to notify her on the usually rare occasions when I take/make vanilla calls.)
There’s also, generally, that your time isn’t yours. If your M-type wants to make you late somewhere, or wants to make you wait in position to be unleashed from the bed for half an hour (or forgets you’re there)—that’s their choice, and you (probably) don’t get to complain about it much. You learn to work with it.
I maintain pretty much one good friend I see independently of Mistress, and there are plenty of times I have to see them in person at our house, so I can keep an eye on a slow-simmer dinner or talk while I do chores, and I might get called away to go do something like fetch coffee. And one household of vanilla family. You might find a lack of time for huge circles of independent friends, but many people are happy without that in general.
When I had a family emergency last year, I was officially put on (and later taken off) “light slave duty” to allow time to see to that. That might be a possibility in emergencies for many others.
Honestly, it’s mostly the routine service stuff that adds up more than projects. My morning routine (which I’d need permission to really modify) can take up to two hours, including exercise, throwing on my uniform, making the bed, little stuff like that. Not to mention evening routine. Dinner usually takes my attention for about an hour and fifteen minutes for cooking (either fairly active or at least having to keep an eye on it), half hour for eating at the time it’s required to be on the table, more assorted time for cleanup after. I’d count on the fact that if all a day entails is your daily list, it doesn’t mean it’s a super light work day.
And of course, that gets easily thrown off by getting sick or needing some extra sleep. Even if you get/have permission to not do them that day, it still means (likely) that they’re not getting done, and then it becomes a problem for Tomorrow You. This will end up on your mind when it’s at all questionable if you should ask for the rest, and you’ll have to know your own needs there.
I often say, “It comes out to very easily about/more than a full time job, and the fact you’re permanently on call, and no weekends/holidays/etc.” That is, of course, for me and the way my dynamic works, with the note that it would look very different for, say, someone who’s working/pursuing education/etc.
In the end, you’ll learn to prioritize what’s important to you in the free time you have, and how to work that in, whether it’s friends who come to you, or giving up some more mindless R&R time activities to make sure you get in real hobby time.