In my experience, the answer really depends on the wood species, how the bench is finished, and the climate it lives in. If you have a bench made from a dense, naturally oily hardwood like teak, ipe, or white oak, and you keep it lightly oiled or sealed, I would expect 10 to 15 years before you see anything beyond fine surface checking—sometimes closer to 20 years in a mild climate. With cheaper softwoods like pine or fir, especially if the bench sits in full sun and rain, you might see small end-grain cracks within 2 to 4 years, and more noticeable splits by year 5 to 7.
The slatted design actually helps, because the boards can expand and contract independently, but the ends are still the weak point. Most cracking starts at the end grain, where moisture escapes faster than the middle of the board. Sunlight also breaks down the wood surface over time, so a bench that bakes on a south-facing patio will check sooner than one under a shade tree.
There are a few things that make a big difference. First, seal the end grain with a wax or end-grain sealer when the bench is new. Second, apply a penetrating oil or exterior wood finish once or twice a year; if water no longer beads on the surface, it is time to recoat. Third, keep the bench slightly off wet ground and avoid letting leaves or dirt sit in the gaps between slats. Fourth, if you live somewhere with harsh winters, cover the bench or store it in a dry place during the off-season.
Small surface cracks are normal and usually cosmetic. I would only worry about a crack when it runs deep through the board, widens enough to catch a finger, or makes the slat flex. At that point, it is better to replace the individual slat rather than the whole bench.
So, realistically, for a mid-range outdoor bench made from cedar, acacia, or heat-treated wood, I would plan on about 7 to 12 years before cracking becomes obvious, assuming you do basic maintenance. For a high-end teak bench, 15 to 20 years is a fair expectation. If the bench is unsealed and exposed year-round, cut those numbers roughly in half.