try not to think of the gains in actual specific numbers. if you look at it like a gain in a percentage of your cars power, you'll see why the numbers will vary anywhere between 1-30 hp. depending on the intake, most people will probably expect anywhere from a 1% to say 4% increase.
lets say we stick to a 2% increase. an engine that outputs 100hp at the wheels will gain 2hp, where an engine with 500hp at the wheels will gain 10hp.
the results will also vary on the design of the factory intake also.

if we're sticking with hondas for the most part, an intake alone will make a minimal difference, unless you REALLY understand how your car performs and know if any little thing is wrong by how it performs.