I've often wondered that myself. I know that I blew through somewhere around 2400 rounds in about a week in 2003, all through an M-249. After that I kind of lost count. I would estimate in a month (+/- a few days) somewhere around a total of 5000 rounds. Taken into account the circumstances and all, that means that in hard combat the average service life of an M-27 is +/- 3-6 months.
There are all kinds of variable to that of course, but it did get the wheels turning. I am sure that it is more robust than that, and I also know there is very little comparison to a Light Machine Gun with changeable barrels and an automatic rifle, but it is an interesting thought process. After all, the primary job of the Automatic Rifleman in the USMC Fire Team is suppressing fire.