My question is: How fast? How much resistance? Do I do it till my heart explodes and then next time, back off a little?
I would say, go as fast as you can on the bike for those 20 or 30 seconds (what ever interval you decide) and then pedal at a regular pace for the recovery. Use a moderate resistance, around the middle of the road, or even a tad easier. Each subsequent interval will naturally become slower and slower.
For example, I do my intervals on the elliptical machine. For my first few sprints in a session, I get up to 270+ rpms (or whatever the measurement is) and the last few sprints in a session are closer to 220/230 rpms. But even those 220, I feel like I am going all out.
I imagine your body would stop you before your heart would explode.

But I sure feel like its close when I do sprints.