About a week is all I'd count on. Paints are formulated to be stable in storage. When you change the ratio of solvent to binder and pigment, you upset the balance between components. This is true of all paints, not just acrylics. The best plan is to thin only about as much as you think you'll use in a given session.
You may get longer shelf life, but there are too many variables to give you a more definitive answer. After I week, I wouldn't chance it.