That's really tough to quantify. My activity log time is probably a mess considering how much idle+leaving gates open time I've accumulated without actively playing. I will say that the most difficult ones to get were the 5k fish one because fishing is slow and miserable, 1k diving, because it's diving, 5k island medals, which basically was a lot of grinding/waiting for elite scavenger hunt, and the money one, because milkham and I combined our money and it took forever. These days I would say the visitors and visiting ones would be pretty tough since less people are actively playing.
I think I did the island ones about every night for a week and a half maybe? Fishing maybe a bit longer, I was catching 200 a night (12 full inventories+8) and stopping. Bugs were pretty easy since I had caught a lot from stocking up on money. It really wasn't that bad once I set smaller goals for myself. Diving, for example, I got the silver for without really thinking about it. When I wanted to get the gold out of the way, I would catch 100 a night, so just 8 nights of that (actually it was a bit less, but you get the idea). It also helped to do something else at the same time so I didn't lose my mind lol.
