When to take the GMAT is almost entirely gauged on when you want to go to b-school. The scores are good for five years, but if you still have a couple of years of undergrad left and then log 3 plus years of post UG experience before b-school (which is recommended), you should probably target taking the GMAT sometime after you graduate college. If your plan is to go for some of the early option programs, such as Yale Silver Scholars or HBS's 2+2 program, you should plan to take the GMAT before you graduate. Either way, you will always have the five year window if plans change, but if the time lapses, you would need to take the test again. As for prep, you should count on at least 6 weeks of prep time and some take even more. Guided classes are highly recommended, so make sure you budget your time around when classes are available. Hope this helps.