Will,

All mortises here are 30mm. Eventually you might find someone building in really heavy timber or building something where the tenon might be forced to work in traction that might push it larger but I've never seen nor heard of it in practice. Neither in modern construction, nor in any of the older buildings that I've worked on. The 1/3 rule tends to stay in the domain of menuiserie (joinery). In carpentry it's 30mm all the way.

And while the 30mm standard developed at a time when most timber construction was in 15cm square oak, since the dawn of the industrial era and milled pine availability there have been 30mm mortises in 75mm (and sometimes, although more rarely 63mm) timbers with no real problems... except that we feel a greater need to maintain the peg hole alignment apparently! :)-