Reading the description of the Heroes' Birthright virtue, I noticed that it allows for effects that are always active (I assume it would be true also of Mythic Blood). However, there are no indications as to what duration should be chosen in the effect's design to achieve such a result.
Following the enchantment guidelines, I guess Concentration + 5 levels would be reasonable, though since 'always active' implies that it can't be turned off, maybe it should be a bit lower level.
Then there's the question of whether an effect that's always active would cause warping or not. There is a precedent (familiar bond powers) for similar effects not causing warping.
What do you think?