I'd say 5 feet is about the limit. Sorry to disappoint! I assumge you are referring to the Microchip MCP9701 Linear Active Thermistor:http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en022290
The trouble with long wires is that they not only have resistance, but they also pick up noise, particularly 50 or 60Hz power-line noise. To make a longer cable, you might try using a screened type, and add a voltage regulator at the far end (near to the MCP9701). The screening would shield the signal voltage coming back from the device.