At $1.50/ea I feel like humanity can afford to replace them like once every 800 years.

From http://www.luckyironfish.com:

"One Lucky Iron Fish can provide an entire family with up to 75% of their daily iron intake for up to 5 years".

So we'll need to replace them a bit more often than that (not that I'm saying it's not a good cause).

Back of napkin math says we need approx 8mg/adult/day, say we're cooking for a family of 4, we'd need 32mg of iron leached from the fish per day. I'll guess from the picture that the fish weighs about 3/4 of a pound as pure iron, or 453593mg, so it should last about 38 years at that rate. So I was off by a bit. :) Surface area would obviously reduce as it dissolved and the rate of leaching iron would underserve the family before it really began to look unfamiliar, so you'd not want to completely dissolve it. 5 years is definitely overly cautious, it should easily last 2-4x that long, but gives them a good excuse to reinforce the reasoning of the fish when a replacement is provided.