How brewing works
Almost every potion starts as a water bottle with nether wart, which makes an Awkward Potion; the effect ingredient comes next. Weakness is the one that skips the wart: a fermented spider eye straight into water. Then redstone dust makes the effect last longer, or glowstone dust makes it stronger and shorter, never both. A fermented spider eye turns some potions into their opposite: Night Vision into Invisibility, Swiftness and Leaping into Slowness, Healing and Poison into Harming. Gunpowder makes any potion a splash potion, and dragon's breath turns a splash potion into a lingering one.
Times
Each potion lists its time as a drink, a splash and a lingering cloud. In Java Edition a splash potion lasts as long as the drink, less the further you are from where it lands; a lingering cloud gives a quarter of the time. Healing and Harming are instant, and in a lingering cloud they work in pulses at half strength. The four potions added in 1.21 (Wind Charging, Weaving, Oozing and Infestation) last 3:00 and cannot be lengthened or strengthened. Checked against Java Edition 26.3; Bedrock Edition uses the same recipes, but some of its times differ.
Planning a batch
Add the potions you want and how many. The planner brews bottles that share the start of their recipe together, three at a time, so 6 Potions of Strength II take 2 nether wart, 2 blaze powder and 2 glowstone dust: three steps of two brews. Each brew uses one of its ingredient whether it holds one bottle or three, so plan in threes. One blaze powder in the fuel slot lasts 20 brews; the total blaze powder adds the fuel to any used as an ingredient or in magma cream, and a blaze rod makes 2.
Questions
Why does my brewing stand not take the ingredient?
Each ingredient only works on certain potions. Most effect ingredients only work on an Awkward Potion, so add nether wart first. Redstone and glowstone do nothing to a potion that already has the other.
Can I brew the Potion of Luck?
No. Luck exists in Java Edition but has no brewing recipe; it only comes from commands or the creative inventory.
What does a fermented spider eye do to an extended or level II potion?
It keeps it where the game has a match: long Night Vision becomes long Invisibility, long Swiftness becomes long Slowness, Healing II becomes Harming II. Swiftness II and Leaping II have no corrupted form.
How many bottles do I need?
One per potion. Three glass blocks make three glass bottles, and you fill them at any water source.