I'd say to heat seal them and then weigh them with a scale that's accurate to within a gram and mark it on the bag. Weigh it again in 3 months, 6 months, and a year. If it hasn't dropped weight in a year, I'd say you're good, but would recheck once a year to be sure that the bags haven't developed a hole.
I'm conducting a controlled test of short term storage in ziplock mylar bags. Identical quantity of the same tobacco stored in a bag and a jar. It's only been a month, so it's too early to report results. My question isn't about the mylar, it's about the integrity of the ziplock. I use small mylar ziplocks to store tobacco in both my cigar lounge locker and in my floor-standing vintage Dunhill humidor. Both of those offer secondary protection again dehydration. In fact, in my locker, the bags are probably protecting the tobacco from hydration.