Which turn should you go to war against the xeno ( on 4X )
My strategy always focuses on having a solid economy before war. War will beat the hell of your economy and may leave you in a lurch.
Building up a strong Mining and Trade based economy can help you sustain a long engagement.
Also, think of war as a series of goals. Solidify a goal in mind, and then go after it -- such as a taking a set of 3 planets. Prepare for it (have the colonizers ready), execute it (drive back the xeno and invade their worlds) and then try to get back to normal footing for a bit (colonize the worlds, and focus your attention on righting your empire for a while). Rinse, repeat.