[Tagging] Ferry frequency

[Richard Fairhurst]

I'd like to tag approximate ferry frequency in OSM. It's important for 
routing: something that runs every 10 minutes is likely to be useful for 
routing purposes; something that runs once a day, less so.

Before I go ahead and JFDI, has anyone done this / seen this done, and 
if so, what tags did you use? The frequency= tag appears to be denoted 
in Hertz which is not so useful for a ferry. ;)

cheers
Richard