1) I try to donate roughly $1 per hour of content I consume. For example, a podcaster I support creates several different podcast series on different topics; I am interested in two of the series each of which has about four 1-hour episodes per month; so I contribute $8 per month. The $1 figure is pretty arbitrary. A major-cable-network television episode purchased from iTunes/Amazon/Google costs $2-$3, so $1 seems fair for an amateur podcast?
2) The total amount so far has stayed small enough that I haven't needed to set a budget. If I needed to, I might ask myself questions like: whose content would I miss the most if they stopped making it? Which creators do I know to be trying (reasonably plausibly) to make a living from their creations, vs. those doing it as a hobby who would probably continue without Patreon contributions?
3) I have reduced or stopped contributions when I stop enjoying (and therefore stop consuming) a creator's content, or when the amount of enjoyable content they produce goes down. (I know the latter reason for reducing contributions can create a vicious circle, I don't have a good answer for it.) If the person is trying to make a living off their art, and they aren't a megastar, they will never really stop "needing" contributions from some minimal number of fans.
Some creators I support don't easily fit into the above framework. For instance, for Every Frame a Painting I contribute $1 per video. His videos are rare (one every few months), far less than an hour long, make over $7000/video (so I don't feel like my personal contribution is particularly "needed"), and (so far as I can tell) are not a source of income he depends on. But I enjoy them so much that I often find myself watching them repeatedly and find I want to contribute.