I highly doubt wrestlemania; Vince wouldn't want to kill Roman's big moment after he built him up for so long. Perhaps extreme rules after the authority puts him in some crazy match?

At Wrestlemania

At WrestleMania after the Authority and (big surprise) Mark Henry attacks Reigns after he beats Lesnar in a brutal 25 minute fight.

RAW after Wrestlemania.

He will cash in at Wrestlemania 31 after losing to Randy Orton and he will cash in on Roman Reigns for the title,which will then make him one of the biggest heels in the WWE today.

Post-WrestleMania Raw is a good guess.

I expect big things at Wrestlemania or Post RAW!.

considering Vince is not in charge HHH is it could happen at Maina

Extreme Rules would be my guess.