Based on my understanding (which admittedly could be flawed), the ramp test always starts at a given % of FTP and steps up 6% per minute.
I always test in erg mode (so I’m not deviating from the target wattage) and I’ve noticed that failure usually occurs around the 19 to 20 minute mark. Now, since my next test is always based on my previous test’s result, I’m thinking there must be a point in my next test which would result in value equal to my current FTP, and any time spent in the test thereafter would equate to an increase in FTP. I’m guessing that point within the test probably occurs around 20 minutes, meaning that as long as I’ve beaten that time, I will see an increase in FTP.
Obviously this could all be wrong, but I’m still curious if there’s a way of knowing how much time I need to spend in a test to see an increase in FTP.