Group length isn’t administrative. It follows from what the group is trying to do, and knowing the shape helps you tell quickly whether something fits.
Six to sixteen weeks
Skills and psychoeducational groups. There’s a curriculum, it takes a fixed number of weeks to deliver, and then it ends.
CBT groups sit here — typically eight to sixteen weeks with measures at both ends, so you can see whether it worked.
Good for a specific nameable problem. The commitment is contained, which makes it the easiest format to say yes to, and the ending is planned rather than negotiated.
Six months to a year
DBT skills groups run about six months for a full cycle and standard DBT asks you to go round twice. The second pass lands completely differently, because by then you’ve tried the material.
Many time-limited process groups run a year, with a defined start and end and no new members.
Open-ended
Long-term process groups, usually open, with people joining and leaving on a rolling basis. Leaders typically ask for six months to a year as a minimum commitment, and people frequently stay two or three years.
That sounds like a lot and it’s the format where the deepest change happens, for a reason worth understanding: the useful material — how you’re with people — takes months to surface, because for the first stretch everyone is on best behaviour.
The pattern inside any of them
Groups move through recognisable stages, and knowing them prevents the most common mistake.
Forming. Polite. Everyone agrees with each other. The group feels pleasant and slightly unreal. Weeks one to four or so.
Storming. It becomes real. Someone says something honest, or two people disagree, or the group turns on the leader for not running it properly. Uncomfortable and necessary.
Norming and working. The group develops its own way of doing things and gets on with it. This is where most of the value is.
Ending. Handled deliberately in a good group, over several sessions.
The mistake is leaving during storming, which is exactly when people most want to. That moment deserves its own page.
How long should you commit?
Whatever the group asks, and take the minimum seriously rather than treating it as a formality. Groups depend on stable membership, and a member who leaves at week six costs everyone else something.
For yourself: four sessions before judging any group. One is unrepresentative in every format, and in an open-ended group the first month tells you almost nothing about what it will become.