police clearance expired.. need for resubmission?

Relevatively short answer: any Visa Office may request (require) an updated police certificate from any applicant at any time during the processing, right up to the issuance of the visa. It is my impression that different visa offices have their own policies (or at least prevailing approach) about if and when updated certificates will be requested; some, it appears, routinely ask for them if the application has been in process for a very long time. I do not think there is a fixed date. I wonder whether they might limit the request to only those countries the person has been in since the issuance of certificates that were submitted with the application.

Bottomline: if your app has been in process for more than one year, there is a fair chance they will request updated police certificates.

More explanation than you probably want:
For the most part, the idea that police certificates "expire" is a bit misleading (except, perhaps, for some countries which do issue certificates with "expiration" dates; my understanding is that most, by far, do not), since they are generally valid and current only as of the date they are in fact issued.

The idea that they expire is rooted, mostly, in that CIC requires police certificates to have been issued no more than three months prior to the date an application was submitted (actual time limitation may vary by region), although even this is not a firm rule (my own was a few months older than that).

While the length of time an app has been in process probably is the single most common reason they will request an updated police certificate, I believe they can and will ask for updates in other circumstances as well. And, again, I do not think there is any magic number (like the one year for the medical) that determines this. They could, for example, see something pop up in the background and security check that triggers a request for an updated certificate.

Again, if it has been a long process (over a year), there seems to be fairly high odds that many (but not all) foreign visa offices will request an updated certificate. If you think this might delay the process (if, for example, you anticipate needing a certificate from a country for which it takes a long time, or is otherwise difficult to obtain one from), then you could obtain more current certificates and have them ready. (Here too it is different than the medical -- for which, as I understand things, you pretty much have to wait for the request because the request includes the paperwork you need to submit to the DHP doing the follow-up exam)