Most taxpayers who file their returns electronically and request direct deposit generally receive their IRS refunds within 21 days. However, a variety of factors can result in longer return processing times, in some cases substantially longer. The most common causes of tax refund delays include:

The return was mailed on paper instead of filed electronically. Filing a paper return typically doubles the expected wait time for a refund.
The IRS needs more information to process the return. In this case, the taxpayer will generally receive an IRS letter requesting the information.
The return did not include banking information for direct deposit, so the IRS must mail a paper check.
The return includes a request for an Injured Spouse refund allocation.

The IRS cannot provide information about refund status until at least 21 days after a return was electronically filed, or six weeks after a paper return was mailed.

IRS Where’s my Refund: https://www.irs.gov/refunds