I don’t think the hiccups themselves can do you any major harm, but in a lot of cases hiccups are a sign that there might be something wrong with you such as respiratory, or even brain issues. I think the longest bout of hiccups was about 70 years. I’d have to have hiccups for my entire life!
I’ve heard about people who’ve had constant hiccups for like ten years and whilst I think that is very annoying, I don’t think that hiccups are dangerous per se. They could be an indication of an underlying health problem that might be dangerous though!