Testing time is probably more closely correlated to feature scope than development time. I'd also argue (perhaps controversially) that testing time is correlated to the skill of your development team.
For a 6-to-9 month development effort, I demand a absolute minimum of 2 weeks testing time, performed by actual testers (not the development team) who are well-versed in the software they will be testing (i.e., 2 weeks does not include ramp-up time). This is for a project that has ~5 developers.
