Stimpack boosts are not available for standalone purchase. Stimpacks are usually tied to time-limited bundles.
Stimpacks start to deplete at the time of purchase and will not pause if you are logged out of the game, or if you are logged in but not actively playing. Maintenance does not extend the duration of a Stimpack.
The Co-op Stimpack provides a 50% boost of your combined base and bonus XP values. It does not factor in bounties, such as First Win of the Day or Mutations.