How are adults taught to read and write and how long does it take?
To be more specific, the learners are aged between 15-25, most know a few letters and how to write their names but a few don't even really know what reading is, and a few can read but very slowly. There are lots of them so they'll be split into groups, probably based on ability, but groups will be quite large, ideally over 20 - suggestions for bigger groups would be welcome. They'll be learning in the evenings after a day of manual work, maybe 3 times a week but it could be everyday if that's better.
The teachers have modern level reading ability and experience with teaching such people. Let's say for now that they have enough paper and ink to use although they won't be able to waste it. Also they'll have various reading materials but probably not 'class sets' of anything, nor will the teachers be able to make photocopies etc - if they want everyone to have a copy it'll have to be written out by hand. But on the bright side they do have electric light so no need to worry about that.
So I'm looking for suggestions of teaching methods that could be used and how long the process would take.
Thanks in advance!