I hope that you're referring to a (snail mail) letter from HMRC and not one of the dodgy 'HMRC tax refund' emails that are going around.
As Islay indicates, your question really falls into the 'How long is a piece of string?' category. The tax refund which was due to me earlier this year (and which should have arrived within 28 days) actually took about 6 months to find its way to my bank account.
Conversely though, I've posted a tax claim on a Monday evening (after the last post had gone) and received a cheque for the refund due to me on the Thursday morning of the same week.