Ideally, an AC system should hold pressure forever. Realistically, on
a 15+ year old vehicle like the Vanagon, you are indeed probably
looking at 2-4 years before some leak or other  causes the system to
de-gas. Newer cars probably have a longer life, but I haven't seen
many "afterthought AC" equipped vehicles last more than 5 years. There
are always exceptions though. I had a 1980 Mazda 626 with dealer AC
that was still blowing cold in 1988 when it got totalled. Never can
tell, I guess.

> Some folks I've talked to say that even with an 'all
> new' system (compressor, hoses, dryer, cleaned out
> condenser etc) that the most you can expect is 2-4yrs
> between recharges?