They get up to 5 sessions per week of 2.5 hours, term time only (39 weeks of the year). It stops at the end of the term before they go to school. Some private school nurseries accept the grant and will discount the fees to the value of the grant, but not all of them will.
As sidge says, the NEG grant stops at the end of term before they enter school (although in this area it is only for 38 weeks a year).
In my area (sorry, should say NW England - don't really want to be specific about the borough) we have private schools that do access NEG funding and private schools that don't (there are criteria that need to be met before NEG can be accessed).
If you are unsure, contact your local Children's Information Service and they will be able to give you a list of all voluntary, independant and private sector settings that are accessing NEG funding.