Its probably quite different for boats laid up for 6 months per year and it would seem before and after layup is normal.
For actively cruising boats 100 hours can slip by in 2 weeks. Certainly on passage where the engine is generating 3 or 4 hours per day. Or just 2 hours per day at anchor.
At $28.49 per 4 litres, and mine takes just over 5 plus oil filter at $9.95 its coming out at $500 per year oil and filters alone... If you want to add fuel filters as well...?
Do whats good for the donk but without going overboard for the inboard.
Mark
Hmmm, you might want to think about a hydro generator, that seems like an awful lot of engine running time while on passage. My little towed generator is BY FAR the most productive and dependable independent source of making amps while underway that I've got... All day, all night, as long as the boatspeed is up to 4 knots or so...
For anyone racking up those sorts of engine hours, I think an oil bypass filtration setup makes a lot of sense over the long haul... Most regular oil filters operate within the range of 50-80 microns, whereas a bypass filter can filter impurities as small as 1-2 microns... That's a HUGE difference that is bound to pay dividends over the long haul. This summer has been my first longer trip since installing mine, I'm now somewhere between 150-200 hours since my last oil change, and I'm very impressed by how clean my oil still is...
Virtually all the folks out there on Nordhavns and the like are running with oil bypass filter setups... Steve D'Antonio had an excellent article in the Nov-Dec 2012 issue of PASSAGEMAKER if one can find it, unfortunately it doesn't appear to be available online at the moment...