I did the Blue Creek Cave trip from Placencia and it was a long day. I would say that should be an overnighter unless you fly and land close to the caves--Not Dandriga or Placencia. An arduous day, but worth it.
The Maya think that the cave is the entrance to Xibalba, The Underworld. Yes, I was skeered! We floated out the underground river. Also saw Num Li Punit ruins--very cool but between the two, a lot of windshield time.