I thought the extra time Dave played amounted to three days, not weeks?

Three days was what he had been doing with Terezi, however Kanaya tells Jade that…

It’s hard to say how long Karkat and Kanaya spent Frog Breeding, since they had already exiled the Queen by then, so it may have been like, just half of their campaign? Even then they still cut it extremely close and didn’t even finish it properly. The Kids have less than 24 hours when the Frog Breeding commences, implying Dave did indeed spend several weeks going back and forth just getting Frogs.