It say's "When one of your healing spells critically heals, the target is healed for an additional 15/30% of that amount over 6 seconds."
Tested it out,
1809 crit heal -> 30 regen x 6 (Turns out to only be additional 10% regen, instead of 30%)
Also, there is no buff icon to see when Flash of Light procs. You do get the regen at a bugged value as I said though.