Short answer: As often as you see them!
To witness a Big Geomine you can do so by clicking this blue sparkle icon when it populates on the upper left side of the COIN App Home Page.
You will then see a modal where you can try to be within to first 200 users to witness recent Big Geomines:
Once you have attempted to witness three, you can then refresh three more times every three seconds before you must close the modal.
You will notice the option to witness again will populate every few minutes, as more Big Geomines roll in! Good luck!