Goodman Games

So the players have found the kolbod's secret corridor, killed the guards and then foolishly left the high ground. I can't imagine that the kolbods will do anything but post another guard, and maybe attach a bell to the rope and wait for the second climb (then cut the rope while the players are climbing).

I'm expecting the players to eventually try to "rush" with multiple ropes, but I really don't know how long it takes good climbers to go up rope. Nor do I know how much difference it will make if they pre-tie knots, or even try rope ladders.

At a guess maybe 10' per combat round, which makes it hard for them not to all have the ropes cut. But I don't really know.

I don't want to make it too hard for the players to get back up, but I don't want to make the kolbods too stupid to ignore their own dead, the imminent threat, and the obvious "trap" they can make either!

The mechanics of your question break down like this:

What they are climbing determines the Climb check DC.
In this case, a rope with a wall to brace against (as seen in the old Batman TV series if you follow me here) and a knotted rope are identical at DC 5.
If they forgo the ropes (and the potential cutting of same) and just scale the wall, that's anywhere from DC 10-25. Most common "dungeon" walls are DC 20, with ship's rigging coming in at DC 10. You can adjudicate from there.

Making a successful Climb check is a move action that allows a character to climb at 1/4 speed. By voluntarily increasing the Climb DC by +5, the character can climb at 1/2 speed instead.

A character with a base movement of 30 can cover 7.5 feet in a move action (or 15 ft. if they voluntarily increase their DC). This means in a round they can climb 15 ft (or 30 ft), so it will take them 4 (or 2) rounds to reach the top.

Assuming they make the Climb checks!

Failing the check by 4 or less just means you don't make progress. Failing by 5 or more means you fall from whatever height you achieved, taking 1d6 damage per 10 feet fallen (assuming you land on hard ground).

Remember that armor check penalties apply to Climb checks...it's not easy to scale a cliff in chainmail!
If you're worried about your characters not being able to make the Climb (because they all wear full plate, because no one bothered to put ranks in Climb, or because they just have bad dice luck), you can throw them a bone. Insert a climber's kit as some mundane treasure in the dungeon...it grants a +2 circumstance bonus to climbing and Climb-related activities.
If you want to go crazy, a ring of climbing trumps that with a +5 bonus.

Or, you could just let them take their lumps for not putting ranks in Climb. The skill exists for a reason, and their failure to scale Kobold Wall will teach them a new respect for the movement skills.
Sometimes players need tough love.