Someone said based off of statistics for week 1 of the patch, it’s going to take 500 runs approximately to get a full set of tier 3 grinding it this way and buying raider pieces for a lamented piece.
Let’s say you run Naxx on 12 toons a week (remember it’s a weekly reset), it would take on average 41 weeks of straight grinding all 15 bosses of Naxx to complete the set.
OH… and this is before factoring 2.8 million gold and finding someone on your server who has all 8 pieces already to help you craft them 