The perception checks are based on the stealth checks, which are based on the PCs actions.
From experience with other table-tops (and I believe Pathfinder behaves in the same way),
When a player has stated their intention and approach, ask yourself the following questions:
- Is the action possible?
- Do I need to use the game mechanics to figure out what happens?
- How do I determine random success?
So, when your player describes how to move stealthily in some situation, you decide when the check is necessary, which is usually when the situation has changed. A simple example is moving from cover to cover. The player can be seen several times, hence several Stealth checks are necessary. Stealth Checks is defined as:
You are skilled at avoiding detection, allowing you to slip past foes or strike from an unseen position. This skill covers hiding and moving silently.
and Perception Checks as:
Perception has a number of uses, the most common of which is an opposed check versus an opponent’s Stealth check to notice the opponent and avoid being surprised.
So, when the PCs roll for Stealth, the opponents can notice (actively or passively) the movement.
Consider you have some orc seating near a campfire. Two boulders are nearby and your player wants to move through the shadows to the first boulder, then to the second, and then jump on the orc. The other option would be to just charge straight ahead at the orc, but that would not be possible to do stealthily, so the PC wouldn't even bother rolling.
You would tell him to make an easy Stealth check to approach the first bolder, then another to crawl to the second boulder, and maybe a final third to jump the orc. Therefore, the orc would get 3 Perception checks against the Stealth checks if the orc was facing (or had a chance to see) the move.
In other words, you define what actions require an additional Stealth check and the orc can make Perception checks in response to those.
Another easier situation would be if you could actually move right to the second bolder (and closer to the orc) but without ever being in the line of sight of the orc. The DM could rule that it would be impossible for the orc to see you, and have him roll Perception with a negative modifier, like the one shown in the Perception modifier table:
- Through a wall || +10/foot
which makes the difficulty for the Orc to notice you much harder.
The difficulty and amount of rolls you have to make are ultimately dependent on the actions the PCs execute and the different situations they are facing.