For 8 hours you probably don't have to heat the menstruum at all. But if you want, bring it up to simmer, pour over the herb, cover, let sit for the specified time.
With no other info than you've given, however, my guess is that you're making an infusion of the herb, that will then be used either as an ingredient or as a garnish or sauce. If that's indeed the case, I would pulse the herb in a food processor, add the menstruum (most likely olive oil??), and let it sit. Then strain out the chopped herb. | 
Steep for 8 hours? Question. 
