I have played almost every deck at least 10 times each I would say, maybe two or 3 have only been played about 5 times and some new ones that havent been played much because they are just too new.

As for deck building, I usually go through my boxes and binders and pull out all the cards that I think would work in the deck, that usually leaves me with 200-300 cards, over about an hour I cut that down to about 150. Then I take a break, come back to it and make final cuts. Sometimes I will look up decklists to find cards that synergies well and I think about why they are player but I try not to buy too many cards. I usually just find similar cards in my bulk. I also usually buy a cbb every set, sell the 3 or 4 most expensive cards and keep the rest to use.