Indicates that the circumstances behind a bug report are set to be internally reviewed, or that the feature request contains merit to consider but the decision on its approval or decline requires more investigation.
This sounds like the tag is intended for a limited duration.
It also says:
There are multiple factors that may leave a report under review. There could be consideration of whether the proposed feature or bugfix is feasible or useful. There could be investigations on the complexity of the necessary solution. There could be insufficient information in the feature or bug report as presented. Site developers may provide additional comments to request more information or to provide status updates as appropriate to the status of the review, until such time as the decision for approval or decline is made.
This explains some of the process, and it makes sense that this process can sometimes take a fair amount of time, but it still provides no general expectation of a timeframe. Looking at the list of questions with the status-review tag, we can see that there are quite a few that have had the tag for several years already. If nothing has happened in several years, can it really be said that it is "under review"? At some point, should these not become status-deferred or status-declined?