Viper boas do love to soak, but how much seems to be an individual preferance. I had one that rarely left the water. The two I have now soak for three or four days at a time then stay out of the water for several days.

I keep mine about 2 degrees warmer on the high end of the gradient but your temps are fine. They aren't the fastest food processors. Two weeks between feeding and defacating wouldn't be unusual.

Good luck with her.