How long can my betta go without water change?

Hi. I am a sixth grader and I am going to a summer camp for three weeks. I have a veiltail betta fish and his name is Jake. My mom said that she would feed him but she said that she won't clean him no matter what... so yeah... I was wondering if i clean the tank really well (wash all rocks ornaments etc.) tomorrow (friday) and then leave for my camp on Sunday, do you think it would be ok? i'm also only feeding him pellets. No bloodworms or anything. Do you think he will survive? i really really wuv my fishy and i dont want him to die.. i'm leaving soon on the 26th of june so plzzzz answer quickly!!! oh and how much should i feed him if i don't want the tank to get all dirty quickly?