Yep. And you'll need to clear out, too. Depending on how long your stay, you may be able to clear in and out at the same time. The French tend to be more lenient in that regard. In my experience, the actual time spent in the customs office could be anywhere from 15 minutes to an hour, depending on what's going on there. Add any time spent anchoring/mooring, dinghying in and back, and you get the picture.