A sourdough starter is a live thing and must be fed. When you use it you should replace what you used. The amount you should replace will vary according to need. If you use your starter often or you know you are going to have a heavy demand soon, then you can put several cups of milk and flour; equal measures in it.

If you are not going to use the starter for awhile, place it in the refrigerator and feed it once a week, just a few spoons of flour or milk.

If you forget and leave it in the refrigerator for a long time without feeding it, don't throw it out. Try to bring it back by adding 1/2 cup of flour and 1/2 cup of milk and leave it in a warm place for a day or so.

It is remarkable how these things come back.