Forever. When we say "forever", we mean "until you die", and we're not kidding. Read this portion of a message from one of the SPARC Message boards
for a nice example of why we make that recommendation:
"...someone we don't see round these parts much anymore was exactly in the same position. Apparently the bio-mother worked in finance and realized that banks purge their records after 10 years, including microfiche. She also realized that her ex-husband didn't keep great records and knew that he depended on the bank to do so. She went back to court saying that he was behind $23,000 in child support from 10 years ago (how convenient!) and since he didn't have any proof (bank couldn't provide it) he was out of luck."
Now if THAT doesn't convince you to want to "be buried" with your records, we don't know what will.