EIS shares must be held for a minimum of 3 years, however, you can claim your income tax sooner than that.
Tax relief is available for the tax year in which the shares are issued. But you can also treat some or all of the shares as issued in the previous year and claim relief in that previous year, subject to the maximum £1 million relief limit for the year. You'll find the date of issue of your shares recorded on form EIS3.
NOTE: You can't claim income tax relief until you receive the associated EIS3 form. This will be issued to you by the company directly, or the investment platform through which you purchased the shares.