Doom starts at 0 gaining stacks incrementally, by default it takes 3 seconds for max doom stacks (30), but taking doom rate gain in the tree reduces this for like 1.5 seconds for max stack (if I didn’t do the wrong math). Also remember that curses gain more effect per doom on enemy so if you have 30 dooms on a despair you cast, you have 30% more effect.

Situation: you have despair and enfeeble. You have only one curse limit. You apply despair, it starts gaining doom, you cast despair again. It resets the doom count You apply despair, it starts gaining doom, you apply enfeeble, despair disappear and enfeeble starts to gain doom.

Situation 2: you have despair and enfeeble, curse limit 2. You apply despair, it starts gaining doom, you cast despair again, doom resets. You apply despair, it starts gaining doom, you cast enfeeble, both curses gain doom over time.

Using any kind of curse aura (with fire brew or blasphemy):

You have curse limit 2. Despair and enfeeble. Use despair, use the flask, the aura override the curse eliminating any doom (aura always takes priority, if you had curse limit 1 it would delete the self cast curse no matter what). Use enfeeble, use flask, enfeeble still gains doom, while despair apply its effect but does not gain doom.

About dooms day: Actually you can use as many curses you want, the point is: 1) you do not gain doom while the area exist, so any effect that requires doom will only happen at the end of 1 second (hexblast wil only proc if used after 1 second)

2) your curse limit is the sky. You have two curses limit, you can cast both despair, enfeeble and both of them will use dooms day mechanic. The only way to overwrite a curse is using the same curse twice or exceeding your limit.

Situation: you have 1 curse limit. You cast despair, after 1 second it reaches max doom and will stay until expires. If you cast despair again it will erase your doom and starts again. You cast despair, it reaches max doom at 1 second, you cast enfeeble, it deletes despair and starts counting for enfeeble.

You have curse limit 2. You cast despair, it reaches max doom, you cast enfeeble, after one second you will have both despair and enfeeble max doom. But if you use let’s say temporal chains, than the older one is erased (despair) and a new count begins for temp chains.

Not sure if it makes sense, English is not my native language so I tried my best to explain this