Answer: hard.
Looking back at a session I once did for a laboratory study.
Incremental exercise test: after 2 minutes at 300, 350, 375, and 400 W, lasted 30 seconds at 425 W with a VO2max of 5.35 L/min, hitting a max HR of 174 BPM.
Follow-up supramaximal effort #1: rode for 2 minutes at 400 W and then 45 seconds at 425 W, hitting a VO2 of 5.32 L/min and a max HR of 173 BPM.
Follow-up supramaximal effort #2: rode for 2 minutes at 400 W, hitting a VO2 of 5.35 L/min and a max HR of 171 BPM.
Follow-up supramaximal effort #3: rode for 3 minutes at 375 W followed by another minute at 400 W, hitting a VO2 of 5.34 L/min and a max HR of 174 BPM.