Took me almost 4 months, with lots of falls and damages to my bike (and self-esteem) in between. Funny thing is that once you get it it becomes so intuitive makes you wonder why the hell it took so long to learn.

I ride on the sidewalk pretty frequently, though never in manhattan, and obviously never when cops are around. People double park in the bike lane a lot and when the road is gridlocked I just hope over the curb and roll (very slowly!). I usually check the level of foot traffic and how wide the sidewalk is first.