With a successful Climb check, you can advance
up, down, or across a slope, wall, or other steep incline
(or even across a ceiling, provided it has handholds) at
one-quarter your normal speed.Action: Climbing is part of movement, so it’s generally
part of a move action (and may be combined with other
types of movement in a move action). Each move action
that includes any climbing requires a separate Climb
check. Catching yourself or another falling character
doesn’t take an action.
So the average human (Speed 30) can climb 7.5 feet with a single move action, and go ahead and make that 3 squares (15 feet) with two move actions, each action requiring a climb check. Climbing is slow and hard. A high level monk, on the other hand, with a speed of 80, could climb 20 feet per move action.
A perfectly flat ceiling (your modern house) cannot be climbed by anything. There is nothing to hold onto. The ceilings in dungeons and caves tend to have bracing, beams, decayed out hand-holds, and more that make holding yourself upright possible, particularly for critters with a climb modifier above 20.
Isn't this the rule for climbing half speed (15 feet)?:
Accelerated Climbing: You try to climb more quickly than
normal. By accepting a –5 penalty, you can move half your
speed (instead of one-quarter your speed)
That's in the same p90 of Core Rules.