You will need to install a user script manager extension to install this script.
WaniKani Exact Review Times
On your dashboard, instead of the "Next Review" section displaying ~4 hours, ~1 hour, etc. this script will show the exact time of your next review.
When your next review is half an hour away or sooner, the script will begin counting down minutes. The exact time will be displayed after the "Next Review" text, so if your next review is 29 minutes away at 8:00 pm, your dashboard will display "29 minutes" and "Next Review @ 8:00 pm."
If you would like to configure this script to your needs (e.g. toggling between 12- and 24-hour format), please install https://greasyfork.org/en/scripts/38582-wanikani-open-framework