I have one Pokemon at lv 100.
My Rayquaza,Moonlight.
Took around 4 months.
I usually trained my Pokemon with it,Moonlight beat the Elite Four and Cynthia multiple times.
It also beat Markus,and the Pokemon at the 'Zones(and trainers around there) a few times.
Amazing,my second closest Pokemon is my starter Empoleon,Lucky.
She's at lv 78(?).
I hardly use her though,so my Linoone,Ziggy,will probably reach lv 100 first. Maybe even my Lopunny,Samson.