I bought new bearings before I ventured out last Sunday for my first fishing trip of 2009. I was genuinely suprised, because the trailer bearings looked as good as new, with absolutely no corrosion. I just needed to adjust the castle nut slightly as there was some slight play, and then renewed the split pin.
I make a regular fishing trip to Rosehearty or Cruden Bay (100 mile round trip), and have 2 weeks on the west coast annually (350 mile round trip). So in 2008, I probably had 8-10 trips (launch and retrievals). On the west coast the boat is moored, so I only retrieve at the end of the holiday. The bearings were fitted new last year by myself, well packed with grease, etc.
I switched to Lubrimatic marine wheel bearing grease. Can't remember the cost, probably under £10 a tub. Fantastic product, I can't recommend highly enough.