From joanne's link
Does volunteering count as taking ‘reasonable action’ to find a job?
Yes, it can count towards up to 50% of the time you are expected to be looking for a job.*
This means that:
if you are required to spend 35 hours a week looking for a full-time job, half of this time (17.5 hours) can be spent volunteering
if you only volunteer five hours a week, then you will be required to spend 30 hours looking for work
if you are looking for part-time work, eg 16 hours a week, you can volunteer for up to eight hours and spend the rest of the time looking for work.
* This will not apply to you if you are in the no requirements, work-focussed interview or work preparation category.
Does that mean I can’t volunteer for more than 17.5 hours a week?
There are no restrictions on how many hours you can volunteer, but you will be required to spend at least 17.5 hours a week job seeking if you’re looking for full time work, or half the expected number of part-time hours if you’re looking for part time work, as illustrated by the examples above.