Even the best blacking job with bitumen will only last about 3 years so there's an optimum point at which the effort that goes into it shouldn't surpass the result.
I once went around my boat after pressure washing to prep and remove old flaking paint, before applying 2 coats of Rylards Rytex and 2 coats of Rylards Premium. That's 4 coats of bitumen. I did it all myself over a week and was very proud of the job, however it still only lasted 3 years, so probably not worth all that effort.
As a rule of thumb, when you start to see rust on the waterline you've probably got another year to get the boat out of the water. This assumes normal corrosion and the boat has anodes, not accelerated galvanic corrosion. I used to worry when I saw rust on the waterline, but every time the boat has come out the water (a year after first seeing the rust) I've wire brushed the area and it was just surface rust without pitting.