Lots of variables
We painted our decks because they had lots of cracks and crazing. We used a 2-part polyurethane: Interlux Interthane Plus. A trial of one part poly (Brightsides) lasted about three days - it simply couldn't handle the abuse our decks get, so we went with the more durable and longer-lasting 2-part. The surface preparation, which includes primer coat(s) took care of almost all of the cracks and crazing on our 1981 boat, which had been raced hard. A couple of spots where repairs had been made by the previous owner needed some filling and sanding before applying the primer. It has now been five years since the paint job, and we need to go back and do some touch-ups. It took about five to six weeks to do our 36 foot boat, what with sanding, washing, dewaxing, dewaxing, taping, priming, sanding, dewaxing, priming, sanding, dewaxing, painting the nonskid and flat final coats, and waiting for the right weather conditions. Follow the directions and spend twice as much time putting on the masking tape as you think you should.