I think it's 1 in 6x6x6x6x6x6 or 1 in 46, 656 throws. Each die has a 1 in 6 chance of rolling a 6.
It's been decades since I've played Yahtzee - I don't remember if you get your initial roll and then 1 or 2 additional rolls of the dice. For example, if on your first roll of the 5 dice, you get 2, 4, 5, 6, and 6, you can leave the two 6's and roll the 3 other dice again. I just don't remember if you have one or two more throws to try for 6, 6, 6, 6, 6.
It's been decades since I've played Yahtzee - I don't remember if you get your initial roll and then 1 or 2 additional rolls of the dice. For example, if on your first roll of the 5 dice, you get 2, 4, 5, 6, and 6, you can leave the two 6's and roll the 3 other dice again. I just don't remember if you have one or two more throws to try for 6, 6, 6, 6, 6.