The drier the seed is in storage, the longer it remains viable. Pods of seed just hanging in a barn may last 3 or 4 years. Very dry seed can last well beyond 10 years.
I really don't know any moisture content or relative humidity numbers to go along with this.
My suggestion is to wait until the humidity is quite low, prior to collecting the seed from very dry pods. Then store it in a completely air-tight container. Cooling a closed container will increase the relative humidity of its contents.
For a typical home grower, a tablespoon of seed will be sufficient for a decade or more of planting, so a much smaller container can be used. I use tiny Ziplock bags, 1" x 3" (available in the arts and crafts section of Walmart or specialty hobby stores), for each variety that is refreshed, and place them, labeled with variety and year, into a freezer Ziplock, which then goes into a recreational dry-proof box into which I also place a packet of desiccant. These boxes are stored in a small refrigerator that I seldom open.
When the time comes to use the seed for another season, the chilled container should be allowed to come to room temperature before it is opened, otherwise the cold of the container will draw moisture out of the air, and condense it as soon as you open it.
It's a good idea to refresh the seed for each of your varieties at least every 10 years, more often if the seed or its storage conditions were not ideal. (At ARS-GRIN, where they hold well over 2000 varieties of Nicotiana tabacum, they refresh 10% of their varieties each year.)
In studies of seed storage in India, where refrigeration is sometimes not available to rural tobacco farmers, they found that seed stored as dry as possible would last over 10 years, regardless of refrigeration. We do know, though, that dry seed stored in a laboratory deep freeze (way colder than your home freezer) is likely to still be viable for 40 years or more.
Bob