In a class of 25 students, in a secondary school, when do you decide to move on to the next topic/chapter?

To follow the requirements of the institution, you need to stay on track with a syllabus.
If you certain that the majority of the students conceptulize the unit, it's time to move on.
For the marginal students, you need to find alternative methods of delivering the material to them.
Give them some extra homework, or if you have the time, meet them on-on one to figure out why they are missing the focus of the unit.

When you think that at least60% or more are comfortable with the running topic they decide to move to the next topic.

Mini quizes or tests depending on the topic always help to evaluate class progress. Individuals can then be identified and if further help is needed it can be given to those particular students whilst the class move onto a new topic.