@ Parmnparsley- I was intrigued by your question so I did a little research on hot tub lung. I found a CDC case study of a family in Colorado who had all contracted severe cases of hot tub lung.
It took the family about ten months to contract hot tub lung, but their cases were severe. From what I understood the infection started off as mild symptoms; taking months to become severe.
I would guess that the body's immune system could fight off the bacteria assimilated from one soak. Hot tub lung would probably take prolonged exposure to develop.
The CDC did state that the infection is preventable. The center recommends that you shower before entering a hot tub as well as the preventable measures suggested in this article. Showering will prevent the bacteria from contaminating a clean hot tub.