This depends on a number of factors including how many questions HMRC are seeking answers to, the detail and complexity of the factual and legal issues involved, the speed or otherwise with which the taxpayer and the HMRC investigator deal with the correspondence and whether there are matters that have to be referred for legal advisers on either side for further advice.
It is not unusual for tax investigations to last more than 12 months, and sometimes 5 or more years – especially where tax avoidance schemes are involved. What seems clear is that tax investigations seem to be taking longer, as HMRC become more aggressive in their approach and tend to assume the worst – particularly when it comes to assessing penalties that may be due for any previous underpayment of tax.