If it is accepted that a refund is due, a common way for people to claim it back is to ask for the amount to be deducted from their next self assessment tax statement. However you can also ask for the money to be sent to you either direct to your bank or savings/building society account or by post (post takes longer and the safety of the refund may not be guaranteed).
The length of time it takes depends not only on the method chosen, but also on whether HMRC need to make any additional checks before issuing the refund (if you have elected to have it sent to you). You can learn more, as well as finding some HMRC contact details, here.
For all the above reasons, the time will vary - however, once you have been officially given a repayment date, HMRC should pay you interest if the refund is delayed.