Here's my waterboxer experience:

87 Westy-PO only took it to dealer, only autobahn antifreeze-new heads at
55K mi. in 1995.

another 87 Westy- PO didn't know what phosphate-free meant, had a small
head leak when I bought it with 117K mi in 1997.

Comparison of the above two makes me think that years are as significant as
miles, and phosphate free antifreeze in and of itself is not terribly
important.

87 syncro Westy- 82k mi., has had head gaskets, not sure when.

86 syncro van- bought as project van with thrown rod at 98K, head gaskets
had obviously been replaced at some time before throwing rod.

Bill