From my understanding, you are auto-enrolled into this scheme every year, and have to opt-out each year if you wish to.
Personally, I would prefer not to be told what to do with my money, and have more choice of where it's invested etc.
I've decided to opt-out until 2017, as the percentages are a lot more attractive. At the moment it's 1%/1%, which on my wages is tiny.
it's every 3 years. If you opt out your employer has to keep a record of the date you opt'd out and then automatically enroll you back in 3 years later, you then decide if you want to opt out again, if you do then 3 years later they opt you back in ....... etc etc.
It's a nightmare for employers to keep a track of this among larger workforces, opting out, contributions refunds, leavers, joiners etc.
That's why it's a good time for me at the moment - loads of employers have no idea how/what to do etc - so looking for advice and options etc.