I know that things are set to respawn in 4 months unless you have something like Nymeria's respawn mod, but... is that true for all ingredients and creatures that respawn? I read in a different forum that creatures and NPCs whose duration in the AI section is less than five will respawn in 24 hours. I was wondering if that is true...

From memory ...

The first re-spawn event for ingredients in a new game occurs one month after the game begins. Thereafter it is every four months for all containers (plants are containers) marked to re-spawn in all cells other than the ones loaded when the re-spawn event occurs.

Creatures and NPC's in leveled lists or marked to re-spawn will re-spawn on the first cell load back to a given cell if the corpse was removed or after the first cell load back to a given cell after 72 hours if the corpse was left behind.

So, it is different for different things, that's good to know. thanks for the info.