If the IRS did not give you an idea of when they will be completing their investigation in your letter, we cannot tell you either. It could be a couple of weeks or longer. Usually, if the IRS has a time frame available, they will tell you in the letter.
This is the IRS' strategy to verify identities and combat fraud which has been increasing every year. This does not mean that the IRS is targeting your return in particular. They are screening returns for verification of the data.
The IRS describes these reviews as "pre-refund fraud detection, revenue protection and associated account resolution activities. These activities are a critical piece of IRS strategy to address identity theft (ID theft) and prevent unwarranted refund issuance by stopping fraud attempted through Individual Master File (IMF) return filing."