Obviously, the more prepared you are going into the project -- the more decisions you've made and plans you've completed ahead -- the faster you'll complete the installation. I have been involved in two major installations (not involving either program you mentioned) and one took four months (information site), the other took nine months (information and e-commerce site).
The four-month installation went fairly smoothly but involved only three off-site programmers, one on-site Web developer, and one on-site Web designer. We knew the content, we worked out the design elements, and we made data decisions all beforehand. I was the decision-maker. We had a week for testing.
The nine-month installation was the "project from Hell" that I took over from another project manager. It involved two factory installers who couldn't start work because decisions hadn't been made ahead, plus a half dozen in-house programmers and Web developers who hadn't bought into the project, two Web designers who couldn't get their act together, and a field of marketers who couldn't make decisions. There were lots of delays just because nothing could be begun until basic content and design decisions were available for the installers to work with. Plus the database operation was ancient and the programmers were busy writing workarounds and couldn't provide good feedback to installers. This was a considerably more complicated project as well. To launch we ended up creating our own proprietary system for entering product content (thousands of skus) and then installing the out-of-the-box CMS for the rest of the site content. Hard to believe, we launched on schedule.
Not to complain. I'm merely demonstrating that it can go very easy if things are well planned or very badly if things aren't.
[edited by: GRRower at 12:42 am (utc) on April 14, 2007]