If my airplane is centered, I can see metar weather circles about 150 nm away at the top of my screen and 120 nm to each side. If I drag the plane to the bottom of the screen, I can see them almost 300 nm at the top of the screen. Of course, flying east/west, I could switch to landscape. The metar weather circles disappear at the next zoom level which would put them as far out as 250 nm at the top when centered. Things are getting pretty small at that zoom level so, I'm not sure how visable they would be. This is on an IPad Mini.
EDIT: It does appear the scale changes slightly when switched to vector mode or low enroute. That might be a little bit of a work around to see metars a little further out.