Obviously, from a user rights perspective, the longer the period, the better. Often there's a substantial amount of evidence that has to be produced to conduct these studies and to have these exemptions.
It may be the case that we're not looking at time-limited exemptions at all. Perhaps the approach we would take in Canada would be indefinite, unless there's an appeal or some grounds to challenge the exemption. Perhaps it would be indefinite.
Definitely, a two- or three-year exemption period is probably not sufficient.