I am living with my brother and we are not happy with one another. How long do I have to vacate his house? He does not ask for much money because of my very low income as we both are retired.

Unless your Brother is legally evicting you, there is no time limit as to when you must vacate premises.  I'm not sure what kind of agreement you have with your Brother here or why you feel the need to leave but wouldn't it just be best for you two to sit down and work things out as its no longer cheap to live alone out here anywhere? At least make an attempt to work things out.  Right here appears the best place for you to be now. Good luck