I keep a log of the hours I do.
I change the oil every 50 hours and change the oil filter every 100 hours.
When I got my boat at first I paid a mechanic to service my engine, this was about 12 years ago and was paying him £80.00 for oil change and filter.
I soon got to know other boaters who were amazed that I was being conned into paying £80.
On my next oil change and filter, a friend showed me what to do.
45 mins it took and cost me 3ltr of oil and a filter (£10?)
I now do my own.
It worked out the mechanic was charging me £70 an hour 12 years ago.