How far away from a router can this unit be and still communicate with the router?
That depends on the design of your unit, the antennas used, the configuration, what is in the path between antennas, noise levels in the environment, etc, etc, ...
Google "Link Budget" ...
Yes, that should be easily achievable with direct line of sight.
Maybe not through walls, though - depending on the nature of the walls...
But the nRF24L01 is just a chip - performance is probably more influenced by the rest of the system...
