If a bag of grass seed just sits for months or years at a time, can the seed die? Does the DNA just stop being dormant and rot away?
Stored properly, some seeds remain viable for over 100 years. There are reports of seeds, lasting much longer; 1,200 year old Sacred Lotus seeds from China for example, but claims like that can be difficult to authenticate.
I seem to remember that a little less than half of a bag of grass seen will no longer be viable, after a year, if it just sits on the shelf.
Despite rare tales of seeds germinating after thousands of years, grass seed is quite perishable. In most states, retailers who end the year with a supply of unsold grass seed need to have it tested for viability the following Spring before they are allowed to sell it. A certain percentage is allowed to die, but below some level of viability, they must discard the seed. (Generally, grass seed will carry from one year to the next, but it is not an indefinite process.)