Does anyone know how much before a domestic flight one can drop bags with Delta's skycaps?

We're flying Wednesday and my toddler son is a giant transportation nerd, so I thought we'd take the train to MSP. But, I can't take his bag, my bag and a giant bag of Christmas presents, plus him and a car seat, on the train. So I was thinking I'd drive down early and skycap the bags. My wife will be with us with a car on the way home, so the return is easy.

Thanks for any help.