It's been noted that some players lurk in the heroic realms and attack others who enter the various heroic realm boss areas and attack weaker players. It's happened to me a few times and its incredibly frustrating in a middle of a boss fight for a player to attack you, especially after a difficult boss you're just about to beat.

I understand you take the risk in wilds but with boss fights you need organise your heroes in a formation which may not be appropriate for PvPs and therefore become an easy target.

To answer the OP's question, I think heroic realm bosses regenerate 24hrs after defeating them.