I remember learning in OT school that, when choosing a group duration, 6-8 weeks is the optimal time. People start to become bored of attending, and there tends to be a significant drop in attendance after this time. I wish I could remember the reference for this, but I really have no clue where I learned this, so take it with a grain of salt.
However, I have seen so many ways that this applies to life. When you sign your child up for music lessons or soccer, your child is excited to go and practicing at home for the first few weeks. However, after about 6-8 weeks, the motivation you saw earlier disappears, and you have a hard time convincing your child to go. Winter holidays provides a good break, and some of the original enthusiasm returns for a few more weeks. However, before the end of the season, both you and your child feel like you have had enough. Lately I have been trying to find programs for my kids than only run for a short session (8-10 weeks).
As employees, we seem to need regular breaks as well. Many committees do not meet over the summer, and often take December off for the holidays. Vacation days, long weekends, and the occasional mental health day help us to keep going with what we do.
I have had similar experiences working with individuals. Enthusiasm and attendance drops after 6-8 weeks. When working long-term with clients, should we schedule break weeks? Perhaps we should schedule the break before the client ends up canceling, or not following through with assignments. Would this allow us to work more effectively? Would our employers think this is OK?
Something to think about as many of us are returning to work from holidays, hopefully refreshed for at least another 6-8 weeks :)