On our Nov cruise, charges hit my on file credit card multiple times throughout the cruise. So I would definitely suggest every few days.
@Angeliki19 as a frame of reference we are a Family of 3 (Son is 15). When we did the Very Merry Time cruise on the Fantasy at the end of November (7 night eastern Caribbean), charges to my band were around $1100, my wife's $1400, and our son's $500. My wife and I did a kick off tasting at the juice bar, Remy Dinner, Palo chocolate liquor tasting. All 3 of us did the Sting Ray Adventure at Castaway, Canopy/Zipline, and one other excursion that included Megan's Bay Beach. Our son had a non-alcoholic specialty drink with every dinner. My wife is more into alcohol than me. And that includes the automatically charged gratuities and purchases at the gift shops and pop-corn and specialty buckets for movies.
You can view the charges in the app as you go. So you can have an idea of when to visit the front desk with more gift cards if you didn't put enough credit on your room.
I did try doing the onboard credit using my Disney Visa, as someone said it was eligible for the 6 month 0%, but it went as a regular charge, not the 6month/0% thing.