What is the criminal statute of limitations on dealing cocaine in Massachusetts?

Hello, Injuin.  Massachusetts has a six-year statute of limitations
for most crimes, with certain exceptions such as murder and rape. 
Dealing in controlled substances does not fall within those
exceptions, so it is subject to the general six-year limitations
period.
http://www.mass.gov/legis/laws/mgl/277-63.htm

You didn't ask, but the Federal statute of limitations is just as
important in any case dealing with drug trafficking.  The Federal
limitations period is five years.  18 U.S.C. Section 3282, available
on the Web at:
http://caselaw.lp.findlaw.com/scripts/ts_search.pl?title=18&sec=3282

See also U.S. v. Midgley
http://vls.law.vill.edu/Locator/3d/Apr1998/98a1842p.txt

Hope this helps, and if you need any clarification, please ask before
rating the answer.  Best of luck.

Excellent!