The interstate battery in my 2001 boxster base lasted 10 years, but I'd say anything in a 5-10 year range is good.
A good way to cause them to die an early death (as it seems you've learned, unfortunately!) is to leave them in a fully discharged state for very long. This can cause lead sulphate crystals on the plates (always present when discharged, but usually small), to grow to large crystals, which reduces the surface area of the plates exposed to the electrolyte (reducing cranking current capacity), and which cannot be removed by a normal battery charging process.
The normal failure mode on 12v automotive batteries I've had is when the cells becoming unequalized, or have uneven current leakage such that the cells just can't be practically maintained in a satisfactorily enough equalized state. The unevenly charged cells (six cells wired in series on an automotive 12v battery) result in the inability for the battery to get a full or satisfactory charge.
I picked up a nice AGM battery for my 2001 boxster recently here in Seattle area locally. It's a bit heavier than conventional flooded battery the same size, and costs a bit more, but I've been very happy with the performance and fit. I've had very good experiences with other AGM batteries in several other vehicles. 8-10+ year life would be my expectation. Details here:
Battery good options.
Last car I had to replace a battery on didn't have an OE-sized AGM option available, so its nice that we have battery options for these cars!
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