Claude & OP-
The first big mistake employees make with sick time off for real health issues is they do it, not keeping their employer ( supervisor) in the loop. Bosses need to plan their work too and unless you work in government where often there are 5-6 people doing one person's job, especially small business owners depend on the productivity of each person to meet commitments they have made about schedules and budgets. So, always, keep your supervisor in the loop.
Second- Good employers will often help you through a serious problem requiring time off if the health issue require it, but it is based on a track record. You indicate having accumulated the time so that indicates you have that track record. Work with your boss so he can plan around the time you need and he needs to meet the work load. I base this on the assumption that you work for a fair boss. I understand there are many a**holes out there. I was never called one as plant manager (that I know of) but was often called an sonofabitch for never bending the rules or making special cases out of pitty stories. I would give good employees opportunity and bad ones the pink slip! That made me a sonofabitch in some's eyes and a hero in others. All one had to do to be a good employee for me is do the job they agreed to do, work the hours they agreed to work, never lie, and never steal. Professionals in my charge, Chemists, Electricians, and Plumbers were also required to maintain a competency level of their craft.
Claude's situation is not unusual but becomes a real problem if the business owner allows the practice to continue without consequence. It is called malingering. One thing emphasized in management school is you never tolerate malingerers. Give them a firm warning and if they continue you get rid of them. As is said, a new broome always sweeps clean and no clerk or assistant is that critical to your business that a good employer ( boss) needs to tolerate people who take advantage.
I spent 25 years in plant management and attented several management schools and the game is always the same- The longer you keep a person who has problems, the harder it is to get rid of them. Last year my wife had a similar situation with her group of support staff. I guess being a woman made her more compassionate than I was as a plant manager, but she'd come to me with these stories of this one who scheduled time as you described, Claude. Finally I gave her a little lesson in Maslo's employee pyramid theory of fives. Not sure that is the official name anymore, but it works like this-
You have 5 employees. You rank them in performance. The top two are achievers with the one at the top always in fear of losing his top rank, he works hard and the #2 works even harder to beat him off the top. You reward both well with the one at the top a bonus. The guy in the middle is in the middle because he is comfortable. He won't improve as long as he is in the middle. He feels safe and money won't motivate him either. The #4 position is also safe but he keeps looking over his shoulder, worried if any change happens he may be the next one out the door. He does just enough to keep from being last. The #5 guy is the one who is taking the time off and the one your other employees despise as they have to work harder to make up for #5's bad habits. PLUS, all your employees think less of you for keeping this #5 dead wood around. You decide one day you need more performance. Fire the bottom two and see what happens. All of the sudden that guy in the middle who thought he was safe, is now at the bottom and next to go. He pickes up his performance level and you now get more work out of your team but you don't have enough so you hire two more and the chances that you may get another number 1 or #2 type performer are greater than had you continued to hope the original #3,4&5 would become #1 and #2 performers. My wife went to the member of her team who was always making excuses to leave early and come in late and told her that the job she has can't be done with the hours she needs to have in her week for work time. She should look to finding other employment in a part time capacity; but the job here is full time whicvh she can't manage in her life. She suggested several companies where they operate with that in mind. My wife said it was amazing that after she got rid of that one the other 4 in her team worked harder. Then she hired a replacement who became a #2 performer in the team.

Now the older #4 was at the bottom and also worked much harder fearing they would be the first to go in any downsizing order by the company.
In the end I discovered I could get the plant output to greater levels with a reduction from 75 to 40 people. Plus, those 40 people remaining had a renewed spirit of teamwork I did not see in my plant when I had more employees on record.