Stealth, Concealment, and Perception

[MasterBuilder]

I have a situation where the PCs are approaching a group of orcs, in a medium forest.

The orcs have been there for a few days, so have managed to settle in. This includes concealed guards in the bushes around the camp.

I can't find anything saying how concealment affects Stealth. Obviously, concealment allows the Stealth check, but if you've had the time (take 10/20?), you should be able to set yourself up so that you have a good hiding place against anyone trying to see you as they approach.

Rogue skills like Camouflage (and the same named spell) exist, but come on, you're telling me a thick bush won't give you a bonus to hiding?

[Ryze Kuja]

I couldn't find an exact rule for this, but I imagine it would be highly similar to the Improved Cover rules.

Improved Cover wrote:

Improved Cover

In some cases, such as attacking a target hiding behind an arrowslit, cover may provide a greater bonus to AC and Reflex saves. In such situations, the normal cover bonuses to AC and Reflex saves can be doubled (to +8 and +4, respectively). A creature with this improved cover effectively gains improved evasion against any attack to which the Reflex save bonus applies. Furthermore, improved cover provides a +10 bonus on Stealth checks.

[Zarius]

CAN you take 20 on stealth? 10 is one thing, but take 20 can't be used of you have a penalty for failure... I would assume this includes anything with a vs check.