Put your ear to the jar. Is it hissing? Then it is alive and ready. If no sound and no bubbles but it still smells good, keep feeding it until it starts bubbling again. If it's a new starter, usually better to feed it for a week at least do it gets stable.
I usually use it to make my ginger beer once it is bubbling well, and make a new batch each time. The one time I kept it going longer I ended up with some delicious ginger vinegar (though not good for making ginger beer).