How long can a employer hold a check?

If you were discharged by the employer, you were due your final wages on your last day of work.
http://www.ago.state.ma.us/sp.cfm?pageid=1110

If you quit, your wages are not legally due until the regularly scheduled pay day for the pay period which includes your last day worked. (The same link as above; see the hyperlink for the actual law.)

I agree with Pattymd. Also, MA law, c.149, sec. 148 (http://www.mass.gov/legis/laws/mgl/149-148.htm) requires that,

"any employee discharged from such employment shall be paid in full on the day of his discharge"

and by in full, they mean only with a VERY limited set of permitted deductions, like lodgings and meals (as addressed by the DOS in a 2003 opinion letter), which few employers actually deduct for. Employers can also deduct for lost merchandise, stolen cash and other calculable things, but there is limited guidance on this since a 1970s memo issued by the DOL, but they can't deduct below minimum wage.

Most employers just pay in full, because its cheaper to pay than to fight and lose.

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This is provided for information purposes only. This is not legal advice, and the author makes no representation as to whether any of it is accurate or correct. You should consult a lawyer who can provide advice tailored to your situation before making decisions with legal implications.