I found the following in the rel 9.0 HTRM:
Approximately 5.17 miles (8.28 km) over 22-gauge cable with 300 ohm set, or
3.25 miles (5.19 km) over 24-gauge cable with 300 ohm set, or
2 miles (3.2 km) over 26-gauge with 300 ohm set. -- Minimum 40 Vrms
Hope this helps.