I have worked with my current employer for a long time, sick day allocations are generous, and they roll over. The only "medical" issues I have had have been associated with maternity leave, and those come out of a different bucket - weirdly, I wasn't allowed to claim sick leave while on maternity leave, even though a chunk of my maternity leave was unpaid and the extra would have been useful, but new sick leave entitlements just kept accruing the whole time I was away, because maternity leave counts as "service". I have over 140 days accrued...
I am currently looking at other jobs. If I move within the sector, some institutions will let you transfer the whole balance, most cap the transferred balance at 30-40 days, and some won't transfer it at all. It is not paid out, so if I don't use it, and I can't transfer it, it's just gone.
I have watched lots of people mysterious take extended periods of sick leave - often in the form of 'stress leave' - and then, when that was used up, pop up at another institution. In fact, it's more common to do that not, and I would infer that people already have the other job in hand before they go on 'stress leave', and use the sick leave benefit for a bit of a break between two positions (in my field, it's not unusual to get a job offer several months before the job actually starts - and it's also common to be able to negotiate a later start, even if the job could potentially start immediately: jobs are often filled with people from other states or other countries, so delayed starts are sort of a built-in assumption of the process). My workplace /is/ stressful in various ways, but nevertheless it's so clear that this is what people are doing, that I'm unsure how they are getting doctors to sign off on it. We have to provide a certificate to take more than three days, and some people are taking sick leave for six months: how much interaction with a doctor are they having to take, to get signed off for that long?
At the moment, I'm in a weird position where, for the first time, I actually /have/ been significantly ill this year. I was injured in July, sort of blew it off for a couple weeks, made the injury much much worse, and then was in really serious, work-disrupting, pain for a couple weeks, and follow-up medical care involves medical appointments once or twice a week - and, although things have suddenly gotten much better so maybe the end is in sight, the current medical advice is that appointments will probably continue for at least another six months, and possibly longer. Now, I work from home most of the time and, except for the two-week period when things were really bad, I basically just sit in waiting rooms, answering emails on my phone, in a way that's more or less indistinguishable from my sitting at my desk at home answering emails... Even during the two week really bad bit, I still did essential work from home, including some phone meetings, but I did have to cancel or ask other people to cover some things for which I would normally go into the office.
Still: I put in a sick leave claim for the two-week period, and would be happy to put in a claim for the days I'm hanging out in medical waiting rooms. However, my manage rejects the claims. Not in the sense that he's denying me permission to stay home from work. He just keeps going, "Keep your sick leave for when you really need it". When I've pressed him on this, he says that sick leave is "all or nothing" - that I can't take sick leave while still being available on email, etc. I do understand this, and certainly I think it can be important if someone is more seriously ill than I have been, that they don't feel pressure to reply to work emails. From my point of view, however, my life is easier if I can spend a bit of time keeping things ticking along - it would be a much bigger mess if I were just to have suddenly become totally unavailable, even for things I could still do around illness. I've offered to pro-rate the sick leave claims somehow - to say that, on X dates, I was only available for Y hours - but I've been told that "HR don't like that": that you are either sick, or well, for day-long chunks. I've also offered to tally up the hours I've been ill, and put in a claim for day-equivalent sick time. This gets us back to "Keep your sick leave for when you really need it."
So basically: I haven't been allowed to take sick leave while on mat leave (but took unpaid leave instead). And I haven't been allowed to take sick leave while sick (but was paid as if I'd been at work normally). I feel guilty about somehow convincing a doctor to sign me off for 140+ days on the grounds of "stress" - but I also feel a bit like I'm leaving benefits on the table. I'm happy to walk away and lose the days I think I "ought" to have taken sick leave anyway - I feel like my manager is being weird rejecting those claims - but the balance is so huge at this point that, even if I write those days off, we're still talking months of accrued benefit. I'm not sure how people would react if I tried to take two days a week over an extended period - there is a murky provision that allows HR to check fitness for work if there are a lot of short absences for illness, and I've never seen anyone burn their sick leave that way here, which suggests they might actually be serious about pursuing it.
I also have an excess of /other/ kinds of leave - just "won" a paid sabbatical, and then I have months of annual leave to take - so I can't realistically do much about the massive sick leave accrual in the short term. But this means it'll just keep growing... We do have ageing parents elsewhere in the world, and some of me has been wondering about using up some of this balance as 'carer' leave to travel overseas to spend some time with them but, while they are certainly getting to the point that they could use some help periodically, there's nothing dramatic or acute that absolutely requires us to do this, so I wouldn't know how you'd get a doctor to evidence it.