A ramp test is designed to estimate your FTP. It uses a relatively short progressive build of one minute steps to quickly identify the upper limit of your aerobic capacity. The ramp portion of the test should, ideally, take about 5-20 minutes to complete. It should start out feeling extremely easy. And then, rather suddenly, it will get much, much harder. It is at that point - when the difficulty really shifts - that you need to dig in. From there, you should max out within 3-5 minutes. This is a maximal effort until exhaustion. You are trying to record your absolute best one minute power. So keep pedaling until you simply cannot go any further. At that point, stop pedaling and the test will automatically end and will shift to a nice long cooldown.