Depends on how much you spend. Our first cruise we didn't spend a whole lot but this last one we spent over $600. Looking at our credit card statement it looks like they charge you when you go over the onboard cash credit limit for the length of your cruise. So for example a 3-4 day cruise has a $300 cash credit limit. When I hit $319 they charged me once and then charged me the remainder at the end.
Ps, I always shy away from using debit cards for anything like this because of the chance of them putting a hold on your checking account. I don't know that Disney does that but I always just use a credit card and then pay the balance off when I get back to be safe.
You can also apply gift cards to your account if you are able to get them anywhere at a discount. Our grocery store gives fuel perks so I estimate what we will spend(tips, hair braids, ect...) and apply to the account.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!