I dunno, I guess I'm just more patient than most, but it only took me about a week or so to upgrade my first deck, and that is with the old bazaar sell-back prices which were much lower than they are now. I did level 3 mostly, with some top 50 in there. There were very few rainbow decks and rare farms in the top 50 back then, but even then, I found I could upgrade a card every 1.5-2 hours from grinding level 3.
The learning curve inherent to rainbow deck play and the false gods is steeper than it once was, but it's still a fair challenge with a good reward. The false gods are still by far the best for farming, even if you don't win over 80% of your games. Even if you throw a rainbow deck that vaguely resembles a traditional false god farmer, you'll win at least 50% of your games if it is upped.