It should not take more than a couple of minutes.
Refresh the add-ons page, try it again, and if it still hangs for more than 2-3 minutes, or does not enable, raise it with https://support.atlassian.com - problem is we need to be reading the logs to see what it is doing, and probably clearing the plugin cache, neither of which we can do on Cloud.
There were a couple of other reports of plugin installations hanging on Cloud yesterday, so I suspect there's something wrong with it in general, not just your site.
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