I believe they are actually valid for 10 years, but, as pbearperry said, each department makes their own rules. Just call and ask.
No, the MSP Trainer's Cert is good for 10 years.
The MSP Cert given a student, per MGL, is good forever.
HOWEVER, some chiefs impose their own restrictions under the guise of "discretion" and therefore only the PD you apply to can answer the OP's question on "what they will accept".
e.g. I know that some PDs were demanding that an applicant get re-certified (training cert) for each renewal. Do NOT know if that is still being done in some towns, but it wouldn't surprise me. The chiefs have an unlimited ability to mis-use their authority if they wish . . . in most cases it is cheaper (by one or two orders of magnitude) to comply rather than fight it in court (and end up with an uncertain ruling, as judges aren't exactly "unbiased" wrt firearms ownership in MA).