I noticed that some goo puddles seem to be permanent in my games, and that some powder gangers I turned to goo many levels ago haven't respawned.
There is a known glitch where Goo puddles never disappear, it's possible this is the reason why you never see them despawn.
Most areas stay the way you left it for about 72 in-game hours. There are some exceptions, as noted on this forum post. Generally it seems that outside areas that have random encounters respawn, whereas inside areas don't. Have a look at this Power Leveling guide - It provides a list of many areas where enemies respawn.
If you want enemies to respawn quickly, you'll need to sleep or wait for 72 hours in-game.
Goo puddles don't despawn - it's a glitch/bug which was never really handled.
As for respawning enemies, there's a discussion here about which enemies do respawn and which don't. Mostly, if 72 in game hours pass, respawning baddies will have respawned.