My company has a policy where you don’t roll over ANY leave. Whatever you haven’t used by December 31 is gone. Of course in theory, on January 1 you could use up your entire year’s worth of leave. In other words, you don’t accrue it on a monthly basis though it effectively works out that way; if you use it all in January, and quit at the end of June, you owe them 2 weeks (or whatever) back for the unearned leave you took.
I believe that if you don’t use up the prior year’s leave, that reduces what you’d have owed if you leave partway through the year - but have not tested it. It’s a wacko policy in general: people hoard their leave until the end of the year, then everyone disappears the last few weeks of December, and management complains about how billing is too low that month. Well, DUH!!! People would use it more evenly through the year if they knew they could accrue a bit more in the next month.
Now, you CAN get special permission to carry over some time under extraordinary circumstances. I was put on a project that required a lot of overtime the second half of the year. I was permitted to carry over 2 weeks, but I had to use them in the first 3 months of the following year.
For what it’s worth, we don’t have a common sick and vacation pool; our sick leave is relatively generous, actually. When I had surgery a couple years back, I didn’t have to blow any of my vacation time. My husband had to use a week of his vacation time before short term disability kicked in.
We do get paid out for untaken leave if we leave partway through the year - e.g. if I left at the end of September and had not taken any leave, I’d get paid out for 3/4 of my annual leave.
Anyway: back to the OP: I’d definitely try to keep about 2 weeks in reserve because of “life happens” or spontaneous trips or whatever.