I haven't made the switch to left handed permanently, I started to work on it over the summer.  I noticed some aches and pains that are starting to linger more and more, so I figured it was best that I start working on bowling left handed and avoid having to make the switch cold turkey.

I averaged about 150-160 and that was bowling once a week, for a couple of months.  My guess is that it will take you a good chunk of a season to average 180+.  It might take you less or more time depending on how much you practice, take any lessons, and how quickly your muscle memory kicks in.

Hope this helps, good luck.