The feat is pretty simple, Allen flew from Earth to the Coalition of Planets, and back to Earth.
The Coalition of Planets is stationed on Talescria, in the Virgo Supercluster
The Supercluster is 53.8 million light years away from Earth or 5.08987299e+23 meters
For the timeframe, the gap between Omni-Man's dissapearance, and Allen's arrival was nearly 2 weeks. So 1 month for the total trip seems to be a good estimate
- 1 month: 5.08987299e+23*2/2.628e+6 = 3.8735715e+17 m/s (1,292,084,372)