Maybe once every 5-10 years?
Good question. I don't remember the last time I took them. I know I did when I had my surgery 19 years ago. And I'm sure I've taken them once or twice since then but honestly I don't remember when or for what. Fortunately I'm pretty healthy. I got a cold last week that lasted for a couple days; that was the first one I'd had in years. Now watch - I'll probably jinx myself and end up sick enough for antibiotics!
Antibiotics don't weaken your immune system, they strengthen the BUGs'--especially if the antibiotics are taken improperly.
For an "acute" condition, I took them when I had oral surgeries in the past few years. Before that, I don't really recall. I remember getting them for a sinus infection several years ago. A couple of years ago I had the same symptoms and went to the doc for it; the regular doc was away and the doc covering for her told me to use a Neti pot for 2 weeks and if I STILL had the symptoms she would give them to me then. It cleared up within days so now the Neti is my go-to whenever my sinuses act up. Usually my doctors won't prescribe antibiotics for an illness until its confirmed to be bacterial.
However, I have taken a small daily dose of tetracycline for years to control Rosacea. I would go off of it for a while then resume when the symptoms returned. There has been a shortage/absence of tetracycline in the US for months and I cannot get it, so for now I'm living with the symptoms rather than try other antibiotics.