I think what the OP is asking is how many skill ranks does it take to be "proficient" in a skill.
The problem is that some skills are more demanding that others due to game mechanics, so this means that 5 ranks in skill A would not be an equal level of proficiency in skill B.
Also, how the skills are used makes a big difference.
Take, for example, Open Lock.
For a Rogue character, it is often necessary to keep this skill maxed out so that the character can deal with devices appropriate to his class level. Whereas a town's locksmith could probably get away with 5-10 ranks, plus equipment and ability score bonuses. Why? Because the locksmith can always take 20 (no penalties for failure), and most likely won't need to deal with "amazing" rank locks ever.
Another example is the Ride skill. 5 ranks is enough for pretty much anyone who doesn't specialize in mounted combat. In fact, unless a character is particularly clumsy, one can get by in non-combat situations (those everyday rides about town) without ANY ranks.
Some skills, as I mentioned before, need a lot of ranks to be very effective. Jump, for example, requires quite a bit of devotion if a character wants to try anything even remotely 'cinematic' or above the human norm.
So, back to your original question. Most pre-written adventures and encounters assume that if a skill is involved that the PC with the appropriate skill should have max ranks in it. This is typical of challenges involving Open Locks, Diplomacy, Disable Device, Search, Spot, Listen, Hide, Move Silently, and others.
For townsfolk-type NPCs, I generally go with a 5, 10, 15 rule. 5 being a general worker/apprentice, 10 being a person who makes a decent living off of that skill, and 15 being a supreme master of such.
In my own characters, if I'm using skills to define things that the character should know because of his background, I generally put 5 ranks into things. So, if I have a character who grew up as a barbarian tribesman, he gets 5 ranks in Survival, or if he was raised in a fishing village, he gets 5 ranks in Profession (Fisherman). Of course, this is all dependent on having enough skill points to spread around because of race/class/intelligence.
Without getting too sidetracked, in my gaming group, we give everyone across the board 2 more skill points per level (8 more at 1st) simply because we all engage in putting points into background skills to represent the character's pre-adventuring career, upbringing, or culture.