Anna, I have an old Polar that shows the recovery- you can set it for whatever time you want. You just stay relatively still during the time period. Using 1 minute, which is sort of standard, I'd use 10 or less as too hard of a workout for condition that day, 10-20 as ok, and 20-30 as optimal for base training. I'd mainly use this to plan the next day's workout- if I get a 9 after a really hard workout (and didn't die!), I'd do easier workouts 'til I got a 24 or so. It's also a really good way to keep track of your overall condition. Sometimes a lower number after a relatively easy workout means you might be getting a cold or have some stress or something systemic, and it's a good idea to back off then and figure that out.
When I ran a lot, I also used to use the A.M. HR as a recovery gauge. After you get a baseline, if it's within a few beats of your normal, rested value, then you're good to go for a regular workout, if it's 5-10 beats higher, it's an easy or recovery day, more than that, off. Since HR responds to all types of things in your body, it can also be an indicator of something else that's off systemically. Over time with a log you can figure these things out- like stress, poor sleep, illness coming on, etc. Different opinions on when to take HR (like first thing, or after morning urination, and if so, how long to rest again), but consistency is most important.
I've seen some of the less than 12 and you'll die articles, but never figured out how they related the workout to that. I think they're based on some study with people who didn't work out taking a standardized treadmill test, and following their mortality for a few years. This is totally different than doing workouts and tracking it regularly. If I did a bunch of 72sec quarters on a 90 degree day, and my recovery rate was below 12, I knew it wasn't because of a bad heart...but I did know I was vulnerable and just jogging a few miles the next day or two, until I was recovered.
