I only donate monthly. I have limited funds to allocate for this, and lots of artists I want to support, so I won't support an artist unless I'm already confident that they can provide the content (obviously I don't cancel my pledge the minute they miss an update or go on a break for a bit, but I wouldn't support someone who routinely takes months to get a new page done, or disappears without warning). So I never feel like I'm not getting my money's worth.
I don't support artists just for the rewards, but these tend to influence the amount I give. The only rewards I'm especially interested in are early updates, and some types of bonus material (learning more about the worlds people's comics are set in, extra comics with side stories and cut scenes). Livestreams I'm not interested in, because thanks to timezones, my job, and my own comic, I'm rarely able to just sit and watch someone draw.
For milestones, I just like to see people thinking about the future of their projects, having ambition and ideas for where they're going. Often that's things like increasing the number of updates for their comic, but one of the artists I support currently also has milestone goals for renting studio space, and hiring an assistant.