When I bought my last bow (last year) we did the initial tuning at the shop and then I was told to take it home and put 200 shots on it and bring it back. A few days later I went back and we retuned the bow and they told me to take it back again after another 300 shots. I went back a week later and they did a checkup but no additional tuning was needed.

And from what I understand rain could definately cause your string to stretch if it's not waxed.