If it took me say, 30 hours, to explore* Stonefalls (exploring includes listening to all quests and completing a group dungeon).
Say each area takes 30 hours and there are 17 areas.
Say the starter islands take about 3 hours each.
Say Cyrodiil would take roughly 50 hours (due to it not being as quest heavy as the leveling areas).
30 x 17 = 510,
3 x 3 = 9,
50,
Add them all together..
It then take an estimated 569 hours, or around 95 six hour play sessions (which is about 3 months) to explore all the land ESO has to offer.
*Please note: this does not include pvp, crafting, roleplaying, or whatever endgame content available.
P.S. I had too much fun making this mock-up, haha.