the-witcher-3
I remember hearing about decoctions for the first time in a preview video, I wasn't impressed back then. But I think I might have misunderstood some aspects, so I'm looking for some clarifications on the mechanics of decoction use.
The Medidation Menu is on the Far Right of the Menu.
There are two menus.
One is a menu where you can save game, load game, exit, etc..
The other is where you have your World Map, Inventory and the Meditation Menu. It's on the Far Right of this Menu.
You can see it here in this image:
You can then select the span of time (You can see the current time you're on and select When you want the meditation to end.) and it will fast-forward to that chosen time healing you in the process.
If you're playing in higher difficulties (Any above "Story and Sword") you will not heal during meditation but the game will always warn you about that.
You can destroy Monster Nests just like in Witcher 2. Crafting or Buying Bombs and using them on the Nests.
In the Witcher 3 it works the same way you have to blow up the nests but with some new twists. Before you would go to the nest and press a button to drop the bomb.
Now in the Witcher 3 you can throw a bomb towards the nest or use a destructive sign at it to destroy it.
"look around the chamber until you spot the two smoking mounds lying either side of the pit. As instructed, use either bombs or one of your destructive spells to obliterate them" - Quote from Guide
The specific bomb to destroy nests in the Witcher 2 was the Grapeshot Bomb.
EDIT:
Ghouls Nests and Nekker Nests can only be destroyed with bombs. Get a Samum Bomb or Grapeshot Bomb (Most Common) and go to the Nest and will ask you to press a button to drop de bomb just like in Witcher 2. Some nests can be destroyed by using the bomb and Igni and some only with Igni.
Endrega nests (Eggs) can be destroyed using Igni and so can rats nests.
If you need to replenish your bombs just meditate with Strong Alcohol like Alcohest and they will refill along with your potions.
Best Answer
I experimented a bit myself with decoctions, and found the following about the mechanics of them.
They do replenish like potions do, as already stated in another answer. They also have vastly greater durations at around 30 minutes. But the way toxicity works is fundamentally different than with potions. The toxicity from decoctions doesn't seem to decay, it was set to a fixed 80/100 in my case (without any skills in the alchemy tree).
This means that it is impossible to have two decoctions active at once as it would exceed the maximum toxicity value. It also means that while using some potions is still possible in addition to a decoction, it is severely limited. In my case, only potions with 20 toxicity can be used, as that is all that is left to the maximum of 100. And each potion use did cause a signficant health loss as I hit the maximum toxicity value.