tiggermoth, that depends entirely upon the rules and booking system of the syndicate that you join.
Most syndicates however operate some form of booking system that allows you so-many days in the diary at a time, including so-many weekend days. That usually ensures that most people get their aeroplane, most of the times they wanted it.
Best advice I can offer is before buying a share, have a think about the useage you'd like, then look at the booking diary for the next few weeks/months, and the tech log for the last few months to see what sort of overall use it's getting.
But in general, I suspect that you'll be pleasantly surprised.
G