That really depends on the data rate you wish to use. If your wiring a simple switch input or LED output then you could probably have it go several hundred feet if not more. If however you wished to send serial data then you might have problems after 50ft depending on the baud rate being used. There are driver/reciever chips (used for RS-485 type communications) that can send and receive data for thousands of feet with simple twisted pair wire.
Yes it would be for serial data, specifically I would like to put a RFID reader using the Wiegand26 protocol (I think the baud rate is 9600) in one location and the arduino in a different location using ethernet wire
I was planning on using 2 wires for serial com and 2 wires to inject the power to the reader