The brewing time depends on the size of the pack you're brewing and whether you're using the direct ingredients or not.
If you have all of the direct ingredients to brew a potion they will be used, and the potion will be brewed quickly.
For example: you need to brew a Potion of Plague. You have enough Scorpion Tails, Obsidian, and Potions of Fear. It will take 15 minutes to brew 1 Potion of Plague out of those ingredients, 30 minutes for 2 potions, etc.
If you don't have the direct components, but you have basic ingredients or intermediate components, they will be used to brew the potion. But the brewing time will be longer as you would have to brew the intermediate components first.
For example: you need to brew a Potion of Plague. You have enough Scorpion Tails and Obsidian, but you don't have a Potion of Fear. The Potion of Fear is brewed from Copper and Toadstools which you have enough. The game will offer you to brew a Potion of Plague from a Scorpion Tail, Obsidian, Copper and Toadstools. That means you don't have to brew a Potion of Fear first, then collect it and then start to brew a Potion of Plague.
It will take 20 minutes to brew a Potion of Plague if you don't have a Potion of Fear ready, 40 minutes for 2 potions, etc.
