The Ramp Test is 5 mins at 46%, then 1 min steps increasing 6% of FTP each time. So, if your FTP is 250, your Ramp Test will look like this.
If your FTP is set correctly, then 250/0.75=333 so you can expect to stop the test around 19:30.
Now, let’s imagine that the FTP setting is way too low. Your actual FTP is 250, but you estimate it at 175. The same test will look like this:
The same 333 failure point (assuming you get that far) won’t be reached until around minute 29-30. The issue here is not the first 20 mins, but the following 10 mins where you are at Tempo and above. In the first example, you will spend 11 mins at Tempo and above, but in the second test it’ll be 15. That extra 4 mins at high levels of FTP will be very fatiguing (especially since you’ve already got 20 mins of pedaling in your legs) and I doubt you’ll reach it all the way to the last step.