Longevity is mostly a function of environment. A car that sits outside all the time or is in an area with acid rain, high dust, sappy trees, salt air or overfed Seagulls will need to be cared for much more often than a garage queen.
There is no hard and fast rule but polymer sealants like Meguiar's NXT, Klasse or Zaino tend to last longer than traditional waxes like Meguiar's #16, Pinnacle Souveran, Griot's BOS, Zymol, etc.
More important than the wax used is how the finish is maintained. Regular washing with a gentle automotive shampoo and immediate removal of any severe contaminants (bird bombs, tree sap, bug splats, acid rain, water spots, etc) will significantly extend the life of whatever wax/sealant is used. Conscientious maintenance will greatly extend the life of your paint.
Personally, I'm a big Meguiar's fan. I like the products, the company and especially the people. Their customer service is superb. That said, I believe you'll get excellent results from any reputable companies' products if you use them diligently. I'm confident you'd be happy with products from any one of a number of companies, Meguiar's, Mothers, Pinnacle, Zaino, Autoglym, One Grand, Poorboy's, 3M, Zymol, Griot's, etc.
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