- a Pit Trap is a CR-1 trap (but it's a lazy kind of trap, so let's assume, as PF SRD tells us to do, that it only costs 250gp x trap's CR) so it costs 2500 sp and its crafting DC is 20
- crafting rules basically ask to roll Craft checks every week and sum them up (assuming there's no failed check), the trap is finished when the sum is greather than or equal to its cost in silver pieces divided by its DC, so with a CR-1 Pit Trap, the trap is ready when the sum is equal to 2500 / 20 = 125
- a 1-HD kobold has +6 in Craft (traps)
Let's say for the sake of this calculation that we consider a very helpful tribe of kobolds and they all have the Cooperative Crafting feat, giving a +2 bonus to the crafting check for each helping kobold. I'm also assuming, to simplify, that on the long run, the die will average to 10.5. Lastly, they are not threatened by any adventurers at the moment, so they can work 8 hours a day without worrying about being attacked, being on watch duty, etc... (if the working team couldn't work at peace, their efficient work time should be halved, as mentionned by the Magic Item Creation rules).
Given all that, an average kobold can't make it alone (DC is too hard to reach), it would take roughly six weeks and four kobolds to dig a Pit Trap, or it takes 56 kobolds to do so in only one week.
It feels wrong that kobolds, famously the best low-CR creatures at trapmaking, spend so much time for one of the easiest trap available.
Am I missing something in RAW ? Is this something frequently houseruled ? If so, how ?