Wow, That's really on the spot. That's exactly the same time we saw at our company (average of many projects) a validation/testing/debuging phase takes... when you use waterfall!!!! Lately we've got the period from "last feature added" to "release" in a lot less time (no statistics yet though), this happened after a transition to Agile (we are using a Scrum-like method).
Debugging is the science phase. Science is difficult to schedule because you’re looking for things, and predicting when you’re going to find them is remarkably difficult. Unless you know in advance how many bugs you’re going to find, you don’t have an ice cube’s chance in the Sahara to work out a detailed estimate of how long this phase will take. Here at Fog Creek we’ve learned that for a new release of FogBugz, this phase takes at least 12 weeks, sometimes a little more, and we just leave it at that.
Check the full post here.