Hard copy 781s we're kept for a year. Which isn't all that relevant as SARMS, along with the rest of the AF, doesn't care about PIC time.

Most people I knew (not an airline guy but many friends have gone that route and I hope to join them) used a multiple of the time accrued since AC checkout. As in multiplying all time since AC upgrade by .9, the missing .1 accounting for upgrade flights, etc, when you weren't the guy in charge. When you hand over your times, you list how you calculated your PIC and be ready to justify your method.

I've seen similar techniques listed on APC threads, if you search you should be able to find it.