if you are applying for PR for your wife, then in order for her to officially become a PR, she will need to "land" in canada. there will be an expiration date on her approval paper by which she needs to land by. This means, she needs to go to canada and go through a small landing procedure to officially gain PR status. IF she doesn't land by the expiration date, then she does not officially become a pr and will need to apply again.
Also, note in your application to sponsor her, CIC will want to see a plan for resettlement in canada. if they are not convinced you will settle immediately after approval, there is a risk of rejection. be careful about specifying timelines for resettlement in canada. it's better to use language like "upon approval..."
If you don't intend to move back within a year or so, then it's probably best to wait to apply until you are ready, especially if your wife is a US Citizen - US applications are getting approved in 4-6 months.
Tehcnically, PR status does not expire. There is a residency obligation PRs must comply with in order to maintain status. if CIC finds out the PR is not maintaining residency obligations, then they can start the process of revoking pr. more than likely, this does not become an issue until 5 years after landing.