You could try bathing your snake in warm, shallow water mixed with olive oil to remove the loose skin. I took a gamble with this on my own snake as I have been told olive oil is safe to use on snakes. It worked and left her fresh scales looking shiny and smooth too. The oil does not "mix" with the water but some of it id deposited onto the snake while bathing.

In answer to your first question, a long time as there are lot of posts online from people worried when their ball pythons submerge their heads for ten minutes plus.

Sorry, I realised after I started typing that the post is almost a year old but hopefully it will help other people who find this question.