"The open WP is to be valid for the duration of one year from the date of issuance. Although the PR application should normally be finalized within a year, subsequent requests from applicants to extend their stay on an open WP will be considered on a case-by-case basis."
Did not see anything that speaks to the validity, except the above. I would assume its valid for the entire duration that it is issued. If not, it would have been noted quite clearly I believe, in order to avoid confusion and complaints on behalf of the applicants, which would lose status. Just my two cents there...
So...good until expires is what I believe. Any seniors?