It appears Tesla has no fixed schedule for exchange rate adjustment. The last adjustment was 9 months ago in early August of 2017. At that time the base price of an S 75D dropped from $101,750 CDN to $96,650. A drop of 5.0%.
This was on the S 75D USD base price of $74,500 and represents a drop in the exchange rate from 36.6% to 29.7%.
It has been stable at that level since.