As a swift action, you can expend one use of mythic power and pick any one metamagic feat you know that increases the slot level of the spell by 0 or 1 levels. For the next 10 rounds, you can apply this metamagic feat to any arcane spell you cast without increasing the spell slot used or casting time.
You can also use this ability on an arcane spell you cast from a scroll, staff, or wand. You can't have more than one use of this ability active at a time. If you use this ability again, any previous use immediately ends (though this doesn't affect spells already cast). You can select this ability more than once. Each time you select this ability, the maximum spell slot level increase of the metamagic feat you can choose goes up by 1, to a maximum of 4.
How many spells can you get off with this?
Could you quicken whatever the hell you want for 10 rounds? (It'd cost you 4 of your path abilities, but still...) Or is it the first spell only?
The fact that abilities like this usually say 'the next spell you cast', not 'any spell you cast', the fact that it has a duration of 10 rounds, and the second paragraph talks about multiple spells cast with the ability, which implies (I think) that you should be able to do so with one use.
On the other hand, 'any spell' is singular, seems like it should be plural 'any spells' for the above interpretation. Also, the second paragraph pertains to multiple uses of the ability, so even though it's weird, you might argue that it was supposed to mean you'd already cast multiple spells with multiple uses of the ability, then activated the ability again, didn't cast a spell, and then used it again, resetting ending the prior use without getting a metamagic feat out of it.