Assuming the set follows the same pattern as the last two, there will be 52 commons and 35 rares. As most of the cards you open are common, we can safely assume rares will be the limiting factor in how many packs it takes to get a full set of commons and rares. Assuming you'll need 2 copies of each rare, 70 packs is the average with 1 rare per pack. You'll have some packs with an extra rare and some with no rare (replaced by an epic or legendary), but 70 packs sounds like a fair estimate.
This number also rounds out nicely with legendary probability, with one legendary guaranteed in the first 10 packs and one more on average per 20 packs afterwards for a total of 4 expected legendaries.
The reddit article above that calculated 64 (actually 63.15) packs, assumed 72 rares per expansion (and 1.14 rares per pack which hardly makes a difference) which was the case with Descent of Dragons but not anymore
Thank you to everyone. It was really useful the information you provided. So i need to open arround 60-65 packs. I will have to then estimate how many free packs i think i will be getting from the new track system and buy the rest of the packs on gold.
Don't forget they will also release a mini-expansion at a later date. 35 new unique cards (rarity distribution unknown), which will also be found in the Madness at the Darkmoon Faire packs once it releases.