IIRC the hourly rate is 1/3 of the daily rate, so less than an hour late you're charged 0.3 extra days, more than 1 but less than 2 hours is 0.6 days, more than 2 hours is a full day. So, you should be charged for 1 additional day at the "extra day" rate.
Normally the "extra day" rate is the same as the daily rate on your reservation, but if it's a "weekend day" rate it can be substantially more (the extra day rate should be listed on your contract when you get it).