Life drawing classes can either be taught classes (perhaps at an art school, or by an artist) or groups of artists who just want to come together to practise their drawing.
Note: there’s also one-on-one modelling for a single artist, although this is less common, and should be approached with caution for personal safety.
Usually a class (probably around 2-2.5 hours long) will start with some short poses; either really short (like 10 seconds) or 1-2 minutes (or a combination of both), and the more different the pose the better, as they’re designed to get the artists sketching and warmed up ready for the class.
When life modelling, you can either choose your own pose or be directed by the artists or teachers. Being directed is a little easier as it takes some of the brain work away, but choosing your own poses can be fun.
Then they’ll move into wanting longer poses; anywhere from 5 – 20 minutes. This is where classes may differ – some may stay with different 15/20 minute poses for the rest of the time, and others may want one pose for the entirety of the class (but split into 15/20 minute chunks to allow the model to stretch). Taught art classes tend to do this, so they have maximum time to cover one pose. The group will direct on this, so this is where you need to have a repertoire of poses for different time frames to use on the fly.
Classes usually have a drink/comfort break halfway (anywhere from 10-30 minutes) where you can stretch and have a walk around (with a robe on, not naked, see Life modelling etiquette) and perhaps have a look at some of the artwork (being sensitive to whether the artists want you to or not). If you’ll be carrying the pose on, the teacher or an artist will often draw around you at key points to help you get back into the same position.
Most classes will have some kind of timer for the length of the pose, or the teacher/artist will let you know when the time is up (and sometimes when there is a few minutes/seconds to go) but some of the real short ones (e.g the 10 second ones) you may have to count yourself, moving into the next pose seamlessly.
Some classes will have music (of varying genres!), and some are more chatty amongst themselves than others. Some classes talk to the model, but personally I’m not a fan of this as it breaks my concentration and I find it harder to stay still.
The modelling will finish when the final timer goes off or whether the teacher/artists calls time and you can put your robe back on.