In the early early days, we decided that for most players, the "grind" was the fun part. A lot of players said that they played until they unlocked everything, then quit. That said, there's also the test server. You get everything unlocked, fully upgraded, and can even test new cards/balance changes before they go live on the main server. Unfortunately, most people don't log onto the test server, so you'd have the coordinate with someone on discord ahead of time.
But if you want to play PvP with unconventional decks, the "grind" is still at most a couple hours, all of which can be spent doing PvP.
1. Free PvP decks can be mixed and matched with each other, or even your regular cards. If you build an even somewhat-viable deck, you'll be using at least 10 cards from a regular deck (don't pretend it's a PvP deck if you don't have basics like t1 archers, core spells, etc.)
2. Of your remaining 10 cards, at least 5 will be common/uncommon (because most cards are). These cards cost only a few bfp. Even 5 minutes of queuing PvP will get you 30 bfp, so getting enough bfp for those cards is trivial.
3. For your remaining 5 cards (in the worst case scenario) they might cost up to 2000 bfp. You can sell boosters at a rate of 420 bfp per 1 booster, so 4 boosters should get you pretty much any card.
4. To get the ~20 boosters needed to fill out your deck, you can play every day and get ~1 booster just for playing 30-45 mins. You get an extra 100 bfp discount the first time you buy a booster each day. Do some daily quests and it can be even faster, but we can just assume 1 booster/day.
5. For beginners, you'll get achievements even faster than the daily quests. if you beat all campaign maps on expert one time (not a grind by a long shot), you get 16 boosters. You'd probably double that in other, smaller quests (like, 1 booster for beating any map with deck level 30).
TL;DR playing 30-45 minutes every day would get you over 50 boosters in a month, which is enough to build a PvP deck from scratch. If you use the free PvP decks as a base you should be able to make any reasonable deck in only a couple days. I see lots of players who have been here for ~1 week and have made their own modified PvP decks. It might take slightly longer if you're going for something crazy expensive like enlighten batariel in PvP, but even that can be easily achieved in a month. If 1 week of regular playing is still too much grinding, you can always go to the test server 🙂