The restriction gauge for the average car/pickup is is 25 water column inches of restriction. you can also get the one that tells you what the restriction is rather than a green/orange in case you want to change before 25, but 25 is standard for most small vehicles.
It is measuring total restriction, so if you've added a snorkel or other stuff it's restriction will be considered.
You install it anywhere in the air intake between the filter box and the manifold. When it goes to orange, change your filter and push the reset button.
Comes on Hummers and many Ford pickups that I know of. Electric versions are available, just harder to install as they require wiring to a buzzer or light. This one just requires a hole with a #18 drill bit. No threading, no adapters.