I find it a little easier to use distance to maintain this over time, because I can always see how far ahead of me the car is, but to measure by time takes landmarks and it's much harder to get a real accurate time measurement. In any case, I tend to err a little on the high side regarding distance for this reason.

I give as much space as I possibly can in following distance without impeding traffic, but not less than 1cl/10mph.

For example, if someone merges in front of me at approximately 3 car lengths at 65mph, I'm not going to slam on my brakes to increase the gap, then gun it back up to speed. That's dangerous. I'm going to get off the throttle, maybe brake a bit, and let the gap grow out before reapplying power, which gives me reasonable protection without endangering me too much. This is also much more predictable for cars behind me, which again is safer.

Not every situation is in perfect compliance with the rules. My goal is to maximize my safety, predictability, and compliance without causing traffic problems for others. That's why it's a rule of thumb. It's not written in stone, but it's a good guideline to start.