If you had both Cancer and AIDS, it wouldn't be as bad as this grind.
The mushrooms are obtained from bags scattered around the map. You can pick 2 mushrooms from each bag, and each bag has a respawn timer of about 2 minutes (it seems). You can channel-hop to try and pick them more often, but they're usually camped by players at least in my server.
It took me about 3 hours to get them all. Final results are:
That's RNG, i've just finished doing this one, 156 stale, 13 sour, 5 sweet.
It wouldn't be so bad if it wasn't only 10 spots to share with everyone, and the channel hopping only works when there's high activity (which is when it's often harder to grab them). Be nice if they increased the spawn rate and the number of locations, it's already painful enough with the RNG.
Have fun with the worm heads if you've not done those yet, it's not especially fun either.