Recently, a reader asked me if her levain (sourdough starter) was healthy, because after feeding it looked more like "the surface of the moon" than the bubbling, foaming mixture often described by baking writers (possibly including me at some point). I promised to post pictures of my own levain post-feeding (2-4 hours) for comparison, so here they are:
In this first photo, you can see lots of small bubbles, some breaking the surface. This is a nice, healthy levain.
My next example, from a different feeding a few days later, is less active, but still quite happy.
As to how much to feed the starter: well, it depends. I usually give my quart-to-quart-and-a-half starter about 150 grams each of flour and water, but if the starter seems to little tired, I'll give more (or toss more before feeding). And I try to feed every week. If you're not baking with a levain yet, I encourage you to give it a try. 52 Loaves has detailed instructions, or for a short primer, see this page. Good luck!