I'm lvl 87 after almost 6 months. I'd say 7 to reach lvl 90 if you do all the events (world boss, 2*P wanteds, potions on monday only).

Otherwise it should be closer to 8-9 months.

Until lvl 40-50 you can lvl up every odd day. Then it slows down every 3 days at lvl 60, 4-5 days lvl 70 and sometimes weeks at lvl 80. Maybe a month to go from lvl 88 to lvl 90 at his rate.

Anyways, when you hit lvl 80 the only thing that matters wil be power. So its better to focus on that. Or initiative if you are into pvp.