Answer:
In New York City, 17 to 20 street blocks (north/south blocks) equal a mile, and 5 to 10 avenue blocks (east/west blocks) equal a mile. As you can see, the length of the avenue blocks varies considerably. On the East Side it's more like 8 to 10 avenue blocks to a mile, while on the West Side it's more like 4 to 7 avenue blocks to a mile.
Of course, in Lower Manhattan, these estimations become worthless, since in Lower Manhattan there's no square grid pattern to the streets. They're all tiny and run in zig-zags.