Just keep diggin'.

18 hours is an 'odd' time that doesn't line up evenly with any schedule. Generally speaking, it's 'once a day' and then becomes available when Yoshi believes you've fallen asleep after you found your first one after waking up camping that L40 node hoping for one to be laying on the ground for you when you wake up, dreaming of being visited by the Map Fairy and all the riches it promises you.. but enough about Yoshi's delusional attitudes.

There isn't a timer. If you wish *really* hard, you're sooner to get a fart than a cooldown timer.