I understand that it can take decades for US citizens to petition for and bring brothers/sisters into the US. But what is the typical wait time for US permanent residents to be able to bring their unmarried child (under 21) into US ? A few months? years?

I am not sure that the USCIS processing times shown online https://egov.uscis.gov/cris/jsps/Processtimes.jsp?SeviceCenter=VSC are very accurate.

I also checked the following link
(http://travel.state.gov/visa/frvi/bulletin/bulletin_3258.html)

but I'm a bit lost on the idea of 114,200 visas being available to family based 2nd preference..What does this mean? Are only this many visas available for such categories every year? What does this mean in terms of the probability of someone actually receiving this visa in a certain year (say 2008, if applied in 2008)? How many months or years might it take for someone to actually receive an immigrant visa through this category (US permanent resident petitioning to bring unmarried child under 21)?

anyone have any experience/idea?

These days it takes many years.
One of my cousins (PR) applied I130 for his family (wife and child) three years ago. It is still pending. Even after approval of 130 you have to wait for visa number to be available for 485. This takes again many years.

Instead of applying from PR status, it may be quicker to apply 130 and 485 after becoming US citizen.
We have no other option.