In my little bit of time on here, it depends on your shedule, really. When I was online for tons of time for about 2 months, I had premium and made around 30,000-ish mush. Now that I'm a bit more inactive, I don't make as much, but a faster explore timer isnt going to help me make more. I take the timer time to do other things. Now, if you plan on being active in explore without premium, consider buying a move expansion or two. That's about the only reason I'm still managing, I've bought a few of them. Other times where I've had it and my activity is what is now, I did make a bit of profit from it.
So, if you're active you can get a lot of mushrooms from it, but it seems to earn back your apples even if you're not too active.
That's just my explerince, and I haven't been on here for an extremely long time or anything, but I've had premium a couple different times. Always got the 3 month one