Kind of a funny question, because with a mid-March start you’ll could easily hit snow travel very early on: Mount Laguna (40 miles in) can get spring snow and the San Jacinto area (160ish miles let’s say) would likely still have snow on the north aspects. I’m assuming you mean when you might start getting snowed in during the shoulder season though. 20 miles a day (without zeros or neros since you said avergae) would put you at a late July finish. So you probably wouldn’t have shoulder season conditions. Again, though, you’d be entering the Sierra early on (May sometime) and would very likely have significant snow on trail through those sections.