If your schedule is very dynamic with no discernible weekly pattern then I think your "weekly" review should be just as flexible. Forget weekends or every 7 days. 4 days since the last review? but you have a nice window of opportunity on the plane? do it. Been 10 days but now the opportunity presents itself? do it.
I think the more important things when you don't have a weekly pattern is to be aware of the energy/mood that you're usually in when you do feel like doing it. Is there a pattern to that? Mornings? Evenings? Also, notice the environments that are also conducive to reviews for you. Maybe plane or train rides? My point is, forget the days and instead look for opportunities in your energy level and environment.
I think it's also important to notice your comfort level with your GTD system. You can go through 4 days of really hectic activity and start to feel icky about your system cause you know its not up to date. Or you could go 9 days of relatively calm period and still be comfortable with your system. When you start to feel uncomfortable with your system then its time to start looking for those energy/environment opportunities.