I have kept 2 sourdough cultures going for years, so when people say that theirs have died or they are tempermental, I frankly dont understand it. The first one I made, was started with flour water and whey. The second one was reconstituted from a packet out of a book entitled Sourdough Jack Cookery, and supposedly it is very old. I try to bake once a week, and actually I will only use one of the starters, 1/4 cup starter sits in the refrig for 2 weeks before I get to it without a problem. Sometimes I cant bake as often, so they have sat longer at times. I just add 2 cups flour and 2 cups water, let ferment overnight, and it is all bubbled in the morning. I reserve 1/4 cup to put back in the refrigerator and continue with the recipe. There is no commercial yeast in these, and I keep them in sealed mason jars. they never expand in the refigerator, but liquid separated and forms on the top. I just stir this in when I take it out. When you open it, it smells quite tangy and sour.
good luck with yours.