I started my character at 4 am central on Oct, 1st. I got to level 65 by farming missions such as Violent Duct and Four Trailers.
By that time, my friend that I had been grinding with had played without me for a few hours and finally got to level 75 to unlock Rooftop Rumble.
I developed a method of parking my tank right outside of the parking garage so that I could save ammo. I would spawn, run over to my tank, fire three shells, and then do it again.
On the very last night before it was patched, I grinded that mission for 6.5 straight hours until the patched dropped and I got kicked offline for it.
I ended up as a level 120 after that. I'm currently a level 208 and I've done it almost all legit, except for 10 levels which I did to experiment with the "Cleaning the Cathouse" thing.
My second character, on the other hand, got from level 22 to level 120 in about an hour and a half off of standing in the marker on one of the Madrazo missions where you have to deliver an NPC.