This hatch generally happens around the end of June to early July here in Southwestern Montana. This can be a rather tricky hatch to completely figure out because the hatch moves up stream at various speeds. If the weather gets hot, the hatch can explode and move quickly upstream. If it cools off, the hatch may even stall and slow way down. Like mentioned before, the trout can gorge themselves on these flies and a section of river that fished well the day before, may fish poorly because the trout are stuffed. Head too far upstream ahead of the hatch and the trout won’t recognize the Salmonflies yet.
There are many factors to consider, and if you would like some shortcuts to figure out this hatch I recommend booking a day with one of seasoned guides.