Minimum 3 hours are required but you should consider 4 hours with considering construction of RRTS on Delhi Meerut Road.
From Shaheed Shthal to New Delhi Metro station will add approx. 40 minutes and 30 minutes additional in Express metro.
In...
more... case you need to reach Terminal 1 then you need to change metro at multiple stations so add 20 minutes extra for that.
I will suggest that leave around 6-7 hours before exact departure time of flight for reaching ontime.