About platinium idk cause I dont used for level up, How much time : nasty was n1 in lvl cap and did it in 4-6h i dont remember well, u can check it in AL announcements First 50 to lvl36.
The expansions dont have a comune time, it depends of developers also depends of ur time zone.
And the best pet for level up its abbadon, I usually level up in new maps , cause give more xp, but some players level up in the traditional place: Watchers Tombs 4