I had a medial meniscectomy just over 2 weeks ago. It would have been done during my upcoming ACL & MPFLr procedures but the surgeon was already 'in there' as part of an arthroscopy and MUA so I had it done.
From my notes:
Anaes. Comm. 1445
In O.R. 1452
Out O.R. 1528
In Rec. 1530
As you can see, it was a very quick procedure but obviously, when coupled with your ACLr, it'll take longer.
Typically, your OS will take a look at the condition of your meniscus and give it a poke with a special little tool. Depending on the position of the tear (if there is one), the type and severity of the tear, he/she will then make a decision as to whether or not it needs to be trimmed.
It does seem more common that the damaged/torn piece of meniscus is removed as opposed to being left to heal as lack of good blood supply to the area prevents healing.
From my own experience and that of a few friends, it doesn't seem to be that big of a deal. Having mentioned that, none of us have had huge portions of meniscus removed. In fact, mine was the largest with 50% of my medial meniscus having to be trimmed.
Remember to ask your OS before your procedure if you have any concerns or last minute questions. I'm sure it'll be fine.