Sorry but how the hell did you manage to get through a 42a in a day?! Are you just skidding down everything? I've done god knows how many uplift days at Inners and Hamsterley and 3 days riding at Fort Bill on a 40a Minnion that still looks fine!
Minnions are pretty bald to start with

Different folk seem to have different opinions on whats a worn tyre. To me a 40/42 highroller loses its edges after a couple of runs...its not done, but its not good. A 60 loses the sharp edge much later.
For me a 42 on the rear lasts a day or two at ft bill/dry inners uplift untill i dont wanna use it any more. A 60 will last much longer.
My last 60 lasted 3 months general riding plus 1 x ft bill uplift, ft bill SDA, Aberfeldy SDA, 1 x inners uplift day, 2 weeks in france (halfway through france it was at a point where the center tread would wear no more, the edge tread was ripped to pieces and the side walls had thread coming out!).
42, great if your an elite/top level expert and/or have the cash to splash, but unnecessary for most folk.
42 all the way on the front though!