I don't know. I would say a minute or less is a short rest period. I wear a HR strap and collect the data, but I don't base my intervals on them. For example, last week I did a set of 6x5' @ 105%, with 1' rest (just coasting or softpedaling). On the 5th interval, I peaked at 181 and then dropped to 153 at the end of the minute rest. That's below my LTHR, but not fully recovered.

And with shorter, even harder intervals, your HR may not come down much, if any. Like with Tabatas, for example. That's 20"on/10"off. When I've done them, my HR basically pegs.