Campbell Scientific recommends approximately 100 ohm loop resistance. This means that with a Type E thermocouple, based on 24 AWG wire, the maximum is approximately 100 feet. It is possible to exceed that distance, but it is not recommended. OMEGA provides a “Resistance vs. Wire Diameter” table that displays the resistance per double foot. See page H-8 of this PDF. Using this table, it is easy to determine how many feet can be run with any particular gauge.