5-7" is correct as U.J. said for nat. gas. Most Gas co. dont supply over 7", In residential gas equip a min. of 5 " must be seen ahead of diaphram type gas valves for them to operate correctly as they depend on a press. drop to work, On commercial boilers etc it is better to have at least 6 " for good pilot press. Pressures over 1lb (28" ) can damage most gas valves,
If you are in an area that still has low pressure inner city (cast iron mains) distribution systems it can range from 4" W.C. to 10" W.C. as there is no regulator. That would be the pressure at the inlet of the gas valve. The main burner on all heating valves is regulated to around 3.5" W.C. to 4" W.C. at the outlet on natural gas. Most gas valves allow for about a 1" W.C. loss through the gas valve. That means anything less than 4" W.C. at the inlet is not going to give full 3.5" W.C. at the outlet. Keep in mind by the way all pilots are on line pressure they are not regulated.
On high pressure system (a regulator ahead of the meter) the pressure out of the regulator is 6" to 7" W.C. depending on location.
ON LP gas pressure out of the second stage regulator is 10" to 11" W.C. and typically 10" W.C. at the outlet of the gas valve.