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.
".. a special power which you can invoke and cancel at will, or which is always active."
The Virtue continues to say:
"It should be designed as a Hermetic effect, no greater than lvl 15..."
As I read it this the answer is as follows.
First, design a lvl 15 spell. That spell, will have a duration. That is the duration of the spell modified by the fact that the Virtue owner can always invoke or maintain the spell effect.
So if you had Fist of Shattering, you could use it over and over.
Strike of the Angered Branch would last as long as you concentrate, but be cast over and over again in definetely lasting as long as you concentrate.
A Rope of Bronze would last until Sun, but could be cast 100's of times in a day.
And so on and so on.
As for Warping.
The rules say that a person must be under a constant effect. First, a person can chose to turn the power off and if they do not use it constantly they do not need to gain warping. If they do use it half the time, then I would follow the guidelines in the book and give one Warping 1 per year of constant use.
I'd think that with something like a level 15 version of gift of the bear's fortitude or eyes of the eagle that I wouldn't warp the character if I were the storyguide. The effect is part of their blood, for them it is as natural as breathing that's just the way that their skin/eyes are.