It never ceases to amaze me how long a seed can sit in the dirt and still sprout. Recently I had a bird of paradise sprout. It didn't last two weeks but it still sprang up and said "hi" for a second.
This morning I had another visitor in a seed starting tray come up. I think it's a bean plant but I'll have to check the charts when I get home.
There are a lot of seets that lay dormant for years before sprouting. Typically seeds last a year or two. You can usually count on most of your seeds sprouting if kept for the usual time for the type of seed. Every year past that you can drop the number by half or more unless you have stored them well. The better you prepare the seeds for storage the better chance you have that they will sprout.
Some seeds seem to sprout on their own. I collected a bag of acorns one year and they sprouted in the bag. I watched it for a week and by the end well over half of the acorns were sending out roots.
My wife planted some coneflower seeds last fall hoping they would come up. Winter came and she forgot she planted them. We nearly removed all of the flowers since we thought we had contracted a bad case of some strange weed! Fortunately we hadn't gotten around to removing them by the time we realized what they were.
Citrus seeds store well since they have a hard outer coating. They can be stored for years and still sprout. Of course after a long period you will want to score the outside or maybe even entirely remove the outer layer so it has a better chance of sprouting quickly.