If it ain't broke don't fix it. If you've got a team of about 10 - 15 engineers that'd be over a million dollars you'd be burning in 1 year for a code rewrite. And most code rewrites that I've seen or been a part of take more than 1 year.
I'd agree with the "more than 1 year" estimate for any large code base. It could even be several years if your run into unforeseen problems. You probably don't want to even start a rewrite like that without first spending a few months thinking really hard about the architecture of the new system and making sure you'll end up with a feature set that does everything that your customers depended on in the old code.
And furthermore, while the developers are rewriting the code, who is going to maintain the old code? The demands for new features and bug fixes aren't going to stop just because you've decided to rewrite the code.
