how long will the seals last on a fresh 727 stored inside a dry building?

It took me 12 years to restore my car.
During that time I had the 727 rebuilt.
I did not put any fluid in it and did not run it at all for 8 years.
During the initial engine break in the o-ring at the dipstick tube was puking fluid.
Simple fix..
The car has been on the road and driven lots since November 2017 and no issues and no leaks with the transmission.

I think if the trans is stored dry (no fluid) and off the floor is should be OK.
Hope this helps..

mine is stored wet but it is on a shelf.its alot easier to replace all the soft parts than to r&r the tranny.but your situation is incouraging.mabey ill just flip a coin.thanks

No worries.
As a guideline,for example, any used transmission my shop sells we are required to replace the external seals prior to installing it to maintian the warranty from the auto wreckers.
Some transmissions from the wreckers may sit for a few years in extreme cases but never had any problems.
Bottom line is...proper storage.
Cheers

Back in 03 I scored a 47RE for my 01 pickup. I built that core using some good parts in the day and have been driving the truck since. I had the OE trans torn apart just to take a peek and threw a seal kit at it as I inspected it since I had it apart anyways. So all those parts have been sitting for 16 years wrapped and bagged in assembly lube and trans fluid just waiting to be put together......

My main concerns would be bugs and critters if it has not been sealed up good, moisture if sitting on a concrete floor, and if it was in a hot building during the summer, I have seen the rubber seals stick to the metal drums and tear when put into service. What I do is pull the pan looking for dead bugs and rust, then I pull the valvebody and air check while spinning the input shaft, if it air checks good I use them as is. At least in my part of the country, seems like bugs and mud dobbers get in everything. I had one just the other day that had been sitting and the low reverse band apply lever rusted and froze, it would not move by hand anymore. Maybe the servo would have broken it loose after fluid got in it, but I took apart and freed it up.

im not near as flexable as i once.Its commin apart for a look see,much easier than R&R.