The length is limited by the bus capacitance. As per the I2C spec, it should be under 400 to 500 pF. The wires and the connected devices contribute to bus capacitance. Use a lower bus speed. For given capacitance, lower bus speed will improve reliability.
The spec can be found here: http://www.nxp.com/documents/oth...
See 17.3 Wiring pattern of the bus lines
I2C is mostly used to connect peripherals on the same board for longer distance consider using RS232 (level shifted, not TTL) or wireless.