This is a boring answer, but assos caps last for ever. Assos completely fails to bump up their sales by making things which fall apart after a reasonable number of years. I'm amazed they're still in business.
For summer, you might want a cotton one, which will cool your head when it's sweat-soaked.
Anyone recommend a good quality cycling cap? I've mostly had Cinelli but they're all wearing out now, would like something a bit nicer for summer.