How often you replace your sails depends on how much you use them and in what conditions.
The jib on the 16 takes a lot of abuse - sweeping past the mast wears on the leech. Probably the worst thing you can do to a Hobie 16 jib (and we've all done it) is being powered up going upwind, and releasing the main sheet before the jib sheet. The mast straightens back up and puts an incredible strain on the jib leech if the sheet isn't eased.
Obviously, consistent heavy air use will wear out the sail faster.
Eventually a worn out sail will manifest itself in an inability to point with your competitors. It will have too much draft, and the leech will be stretched out. A fluttering jib leech is way past being competitive.
I get about 4 years out of a jib with about a 50/50 ratio of heavy to light air. However, I don't do as many events as I used to, and I "share the love" with my 17 and 14, so the 16 only sees 5 - 6 events a year now.
The mainsail has the same issues, but since it's not flogged around like the jib, it lasts much longer. 6 - 8 years of competitive life is not unheard of with moderate use.
Most of the top racers just replace the whole boat at that point, since there are other wear issues (frame looseness, trampoline wear) that start to manifest themselves with a boat that old. You can sell the old one / buy a new one for less than the cost of new sails. It also helps "feed the fleet" - giving a newbie racer a very competitive B-Fleet boat at a significant discount to a new one.