Couple of possibilities, depending on the context...
1. The amount of time that it takes to recover a target— pinpoint it, dig it up, and refill the hole.
2. The amount of time that your detector's automatic retuning takes to recover— i.e., return to the preset or selected threshold level— after something is detected or otherwise affects the tuning
It can also refer to how fast a metal detector recovers from signalling a coin, telling you the information on the target, and being ready to do the same on the next target. My fisher F75 has a fast recovery time. I can swing as fast as I want and get full target information on items inches apart. Some metal detectors recover slower, so you swing slower to get all the information and see if you want to dig the target. The minelab explorer has a slower recovery time in general.
Some machines actually do better the faster you swing, and some do better the slower you swing. Recovery time is good information to know about any detector you are thinking of purchasing, or one that you own.
to me, recovery time is after a heavy night! no that was 20 years ago. after i swing and get a reject chirp it is about one half second before my girl resets her self. sounds fun huh? recovery time.