Just took my CYA Basic Keelboat (Canadian Yachting Association ) exam yesterday. I'm not 100% but Im pretty sure these regs are international.

The regs state that you need to sound your horn in fog at least once every 2 minutes AND after you hear one.

If your sounding more frequently than 2 minutes then you should give adequate time to hear other horns in between your blasts.

Power boats should use one long blast of 4-6 seconds.
Sailing vessels should use one long blast(4-5 seconds) followed by 2 short blasts(1 second each).

FYI _ I don't remeber reading/hearing anyting about the horn frequencies...thus to my knowledge any horn will do.