You may want to use this option to help you determine how many users haven't completed their account activation versus how many have.
Note: This report includes an entry per user per language, so be sure to remove duplicate records before proceeding.
Go to Metrics in the top menu and click on Training
Scroll down to Developer Leaderboard
Click the Export Leaderboard button
You'll receive a confirmation message that the CSV file is being generated and will be emailed when it's ready.
From the exported file, you can sort by status (Enabled, Disabled, Invited, Read-Only) to see which of your users are pending activation and check the invitation date.