i play in a competitive league with two rookies who are new to fantasy this year

anyway, im playing risk/reward with harden, he went 6ip today with 10ks and 1er, looked nasty, to any experienced fantasy player trading for harden is like trying to catch a falling knife, but for those rooks, how many starts can i let him go before i put him up for bait to maximize my gain? im guessing 7-8, any imput?