It depends on a whole lot of different things: what cards you receive, what league you are playing, your skills, and your luck.

I've been doing an experiment these last 10 days: I've started with only a starter deck and no extra money to invest. So to build my deck, I need the earnings of my posts and of the reward cards I can sell.

I'm a pretty experienced player, so up until two days ago completing my daily quest in the Bronze league to get 1 reward card only took me 15 minutes. Considering I got two gold foil cards already, I'd say it is worth the time I put in.

However, if completing a DQ costs you an hour or more, just to get 1 or 2 reward cards.... I think I would consider it a waste of my time.

On the other hand, playing in the Diamond league, I don't mind if a DQ takes me an hour to complete. After all, you get 10-12-14 reward cards in return. And you can have bad luck and get only more Vampires and Adroids and Sea Genies... But there a quite a couple of awesome, powerful new reward cards that make it definitely worth it.