All car hire companies used to have a grace period where if you were up to half an hour late you didn't get charged, but from the travel blogs I read, it seems that most of them have been scrapped in the last few years and they are really cracking down if you don't return it dead on time - especially in the US; with rentals within the EU, there's still some leeway. If you're lucky (like notaprincess), you might be able to talk your way out of it, but I wouldn't guarantee it.
Reading the small print of the documents I got last time I rented with Alamo in the US (late 2010, doubt it's changed that much), they charged you an extra hour if you were more than a minute late, but this hourly rate was one third of the daily rate! If you were more than three hours late, you got charged for a full day. Some companies are more leinient than this, others even stricter.