If concurrent builds are enabled ("Execute concurrent builds if necessary" option) and the job is using postbuildscript plugin, then a build will wait for all the previous builds to finish before it completes.

As an example, say you have a job that sleeps a certain period and then uses postbuildscript to echo "hello world" with "Execute Windows Batch Command". If you now start three builds right after the other with parameters:
#1 sleeps 60s
#2 sleeps 30s
#3 sleeps 70s
You'll notice that builds #1 and #2 actually both take 60s to complete. Build #3 takes 70s. So build #2 waits for build #1 to finish.