Every past build offers information about its spent time. In the build history however only dots are shown and it's not clear whether that mark the start or the end of a build process.
The used widget is part of the SIMILE project and its well capable of showing bars. See example pages http://www.simile-widgets.org/timeline/examples/test_example2/test_example2.html and http://www.simile-widgets.org/timeline/examples/test_example/test.html .
Actually, the build history does show duration for me. Could you provide a screenshot and highlight what you mean specifically?