The more charges stacked together the greater that bonus will be. This is to acknowledge and allow playstyles where you want to prepare before combat, as is repeatedly implied to be the witcher's way in this roleplaying game's writing, in the mechanics of previous games in the series and in all the adjoining witcher media.

Examples:
1x Basic Swallow =20s (vanilla duration)
2x Basic Swallow= 1m (would be 2 x 20s = 40 seconds with the basic version of the mod without the bonus)
3x Basic Swallow= 2m  (1m)
4x Enhanced Swallow= 3m20s (1m20)
5x Superior Swallow= 5m (1m40)

3x Basic Tawny Owl/Black Blood = 3m (1m30)
4x Enhanced Tawny Owl/Black Blood = 7m (3m)
5x Superior Tawny Owl/Black Blood = 11m (5m)

3x Basic Golden Oriole = 5m40 (3m)
4x Enhanced Golden Oriole = 16m (8m)
5x Superior Golden Oriole = 30m (15m) 

3x Basic Thunderbolt = 3m (1m30)
4x Enhanced Thunderbolt = 8m20s (4m)
5x Superior Thunderbolt = 14m (7m30)
etc

Formula: Each time you stack potion charges outside of combat their durations get stacked/added together and an extra bonus also gets added equal to a single charge multiplied by the stack size, up to a maximum of either 100 seconds (which is how long it takes to get rid of toxicity overdose with FCR3) or a singleCharge's worth (if that single charge is longer than 100s).

The 2 minutes you get from one craft of basic swallow  (3 charges) even without any alchemy skills should last just about long enough for the early contract fights in White Orchard. 
Add a modest 3 alchemy skills' worth of duration bonus (45%-75% extra time) and one craft of superior Swallow (5 charges) can last you for 8 minutes, same duration it had in TW1 and enough for any boss fight.

This change means that your preparations won't go to waste if you choose to walk carefully through a monster's lair instead of just rushing to the end to fight the beast asap before your timers run out.
You won't run into any more situations where after inspecting a poisonous monster's clues Geralt says it's time to drink Golden Oriole to counter the venom and then it turns out the monster could only be found 3 minutes later after you potion already ran out. 
No more cases where you start fighting a monster inside a den only for it to fly away to a location a couple minutes away your buffs running out while you're giving chase.
This change effectively removes any disadvantages from making your preparations/drinking your potions before a fight, remove a significant amount of metagamey micromanagement from it.

* Exceptions are made for certain potions such as the ones that aren't meant to be used in combat (wolven hour, drowner pheromones), whose effects are meant to be instant rather than over time (White Raffard's Decoction), or primarily meant to be used outside of combat, for exploration (killer whale, cat).

In order to maintain the same vanilla balance of toxicity vs number of activated potion effects, while you're outside of combat and have active "paused" potion effects your potion toxicity does not drain. Resting or using white honey will obviously clear your toxicity and active effects.