I think the magic number is 370 carries, which Murray is about halfway to already

yes, it's a bleacher report link

http://bleacherreport.com/articles/1271412-why-and-when-do-nfl-running-backs-start-to-decline

Player Year Carries Comments
J. Anderson 1998 410 Blew out ACL in 1999, mediocre in 2000
J. Wilder 1984 407 As good in 1985, but limited by injuries afterwards
E. Dickerson 1986 404 The Randy Johnson of running backs
E. George 2000 403 Fell from 3.7 to 3.0 yd/carry, replacement level since
G. Riggs 1985 397 Fell from 4.3 to 3.9 yd/carry in 1986, part-time player from 1987 on
R. Williams 2003 392 We'll never know now, will we?
T. Davis 1998 392 Blew out ACL in 1999, more aborted comebacks than The Monkees
B. Foster 1992 390 9 games in 1993, 11 games in 1994, then gone
E. Dickerson 1983 390 Ran better than he enunciates
E. Dickerson 1988 388 First full Colts season; finally broke down in 1990
E. James 2000 387 Blew out ACL in 2001, hasn't been same since return
J. Lewis 2003 387 Uh-oh
Player Year Carries Comments
R. Williams 2002 383 Will forever be seen as a fluke year
W. Payton 1984 381 ED-like exception, still great until 1987
M. Allen 1985 380 Fell from 4.6 to 3.6 yd/carry
E. Dickerson 1984 379 The all-time record, 2105 yards
G. Rogers 1981 378 Injuries in 1982-1983, poor 1984 followed by two-year Redskins comeback
E. Smith 1995 377 Fell from 4.7 to 3.7 yd/carry and never topped 4.2 yd/carry again
J. Bettis 1997 375 Fell from 4.4 to 3.8 yd/carry, didn't top 4.0 again until fluky 2001
J. Riggins 1983 375 Fine in 1984, but battled injuries in 1985 and retired
E. Campbell 1980 373 Fell from 5.2 to 3.8 yd/carry, broke down so bad he can hardly walk now
E. Smith 1992 373 Still great until season listed above
L. Tomlinson 2002 372 So far no effects, actually better in 2003 with fewer carries
C. Okoye 1989 370 Part-time player by the following year, done after 1992

another link

http://www.footballdocs.com/running_back_carries.html

I think Murray is a unique situation though

he's had injury/fumble problems

so I'm of the mind set that you might as well run him into the ground while you can