Odot,
You need to be aware that there are two things that affect the lifespan of developers: exhaustion due to use (using up the active ingredient so there's not enough left to do the job) and degradation due to oxidation (which destroys the active ingredient). Both of these are at work when we are using the chemistry. When it's just sitting stored, however, just the latter is at work. You can see that storage is important. If you reduce exposure to air (oxygen), then the shelf life will be proportionally longer. And, you can see that there is a limited number of films/prints that can be developed in a given amount of developer, even if it is fresh.
So, pay attention to the capacity (usually listed by the manufacturer as x-many rolls per liter/gallon) AND to shelf life for whatever storage conditions you have (e.g., "half-full tightly-stoppered bottle").
By the way, these are printed on the package that your D-76 comes in...
One more aside: If you like the look of D-76 but really want a liquid-concentrate developer, look to HC-110. The concentrate has an almost indefinite shelf life and working solutions can be mixed directly from the concentrate without making an intermediate stock solution (using a syringe or pipette to measure the concentrate). HC-110 was formulated by Kodak to give results similar to D-76.
Best,
Doremus