The range onf the M1A1's Rheinmetall cannon is at least 5 miles for the unarmored targets.
It also has this ammo available:
"he new M1028 120 mm anti-personnel canister cartridge was brought into service early for use in the aftermath of the 2003 invasion of Iraq. It contains
1,098 3/8 (
this will hurt) inch tungsten balls which spread from the muzzle to produce a shotgun effect lethal out to 600 m. The tungsten balls can be used to clear enemy dismounts, break up hasty ambush sites in urban areas, clear defiles, stop infantry attacks and counter-attacks and support friendly infantry assaults by providing covering fire. The canister round is also a highly effective breaching round and can level cinder block walls and knock man-sized holes in reinforced concrete walls for infantry raids at distances up to 75 meters."
The rate of fire is at least 5 rounds per minute, so it means that within one minute, the Abrams can hurl
5500 3/8 inch tungsten balls at the charging enemy.
It will also deploy its smoke grenades and it can setup a huge smoke screen within few seconds.
Not to mention the fact that the M1 Abrams can sustain speed that is equal or greater than the speed of a charging horse. It means that the distance between the cavalry and tank
will not change.
Therefore, it all will be over within 5~10 minutes.
Edit: Read about Abrams on Wiki
Wiki Link