Codex: Chaos Daemons page 126 wrote:Soul Sacrifice - Use this stratagem before a CHAOS CHARACTER from your army attempts to summon a unit of DAEMONS to the battlefield using a Daemonic Ritual. That character suffers D3 mortal wounds, but you can roll up to 4 dice rather than 3 for the summoning roll. In addition, re-roll hit rolls of 1 for the summoned unit while it is within 6" of the character that performed the Daemonic Ritual.
Nothing says that the effect ends at any point so I guess it lasts until the end of the battle as long as you are meeting the distance requirement. Kind of unusual, but it would be fiddly to actually benefit from it in play, and Daemonic Ritual is bad anyway.