As much as they give you, minus a few hours. Give the impression that you can meet/beat deadlines, but be sure to utilize as much time as you can and deliver something that's of the calibre they're expecting. For example, if they give you 3 days for a project as simple as this, they probably expect something production-ready: polished, cross-browser tested, responsive, etc. Deliver exactly that in under 72 hours and you should be fine.