Oh boy, my wishlist is shamefully long. I need to give it a trim.

To answer your questions:

I add games that stand out to me, so frequently things that resemble other games I like. Roguelikes and science fiction games do pretty well in this regard. I add games when I see them either in an article like PC Gamer's new games you missed this week, from developer posts of previous games I liked, or from the Steam store front page, so the timing is sort of random, but I probably add a game or two a week (and then purge my list once or twice a year, though I tend to get lazy and quit half way through). I tend to add a lot of games, but the ones that I really want are the ones that are by studios I like, so things like Dominions 5, Metro: Exodus, or 1 Bit Heart are the games that I'm most likely to buy myself when they go on sale- history of past performance helps. However, I mostly use my wishlist to track when games go on sale and I get an email notification. Games like Scythe are interesting to me, but I can't justify spending $20 on a game I might play once, so I put them on my wishlist and try to grab them when they're on a good sale. Usually the summer and winter sales are when I clean my wishlist- if something is 75% off and I'm not interested, it's safe to remove. I don't really bother with rank- with a wishlist as large as mine there just isn't much point to it, unfortunately.

https://store.steampowered.com/wishlist/id/SquireZed/