Impossible to answer and really doesn't even matter. If you don't have work/school, play 24x7, and can live on pizza takeout, it might only take a few days. Within the game, it depends on whether you're soloing, doing any pvp, crafting, exploring GH, making friends and helping them with their quests even though it gives you no XP, etc.
How long does it normally take to get to a lvl 50? How long did it take to you?
I got to lvl 50 in about 3 weeks with my first subscription.