Background:
This is the first time I've DM'd 5e (returning from extensive 2e and 3e experience).
I've run the group through the official "Lost Mines of Phandelver" adventure and just started the "Hoard of Dragon Queen" adventure. All potions the group has looted/purchased have been used since healing ability is a scarcity in this group (Paladin, Sorcerer, and Ranger/Cleric). I only allow long rests outside of active dungeons (Alarm spell goes off alright!).
When the ranger multi-classed as a cleric, she realized she could cast Goodberry with all available unused slots before a long rest. This way, she would have them for the majority of the next day. We are only level 7 now, but I can see the impact of Goodberry getting out of hand (i.e. expending multiple slots with a lot of healing on tap, effectively negating short rests). (I don't allow Goodberry to be used to revive unconscious characters.)
Now, I've found that small brawls/encounters are trickier since the party effectively has pockets of Cure Light Wound on tap (plus the full reserve of cleric/ranger slots). Even the fact that it takes an action for every HP of healing doesn't fully counter this.
The Problem With This:
As you can see Goodberry has changed encounter dynamics — the group can start a dungeon with 100+ HPs of targeted easy healing (only gets stronger as they level).
Hoarding spell slots isn't the problem (wouldn't be an issue in my eyes, this would mean they'd have to not cast spells in other situations). The problem is the prospect of a safe long rest starting with the previous night's Goodberry hoard. To solve this problem, I came up with the following house-rule:
"Goodberry will last 24 hours or until the caster regains spell slots"
This way, you can cast Goodberry before sleeping and it will remain in effect if your sleep is interrupted, but after a long rest it will lose potency (and your spell slots refresh).
Is this a balanced rule change? I cannot find other spells in the PHB that have a similar duration of >8 hours and don't require concentration or active spell slots to compare it with (perhaps Goodberry is a one-of-a-kind spell?).
Best Answer
Your house rule is fine
Our group has been playing with a house rule that overlaps with yours: "Slots used to cast spells that last 24 hours aren't available again until the spell ends", without issues. We started that house rule following some shenanigans with Animate Dead.
But you are ignoring some of Goodberry's description that makes this unnecessary
Part of Goodberry's description is:
So, if you eat 10 Goodberry's that is equivalent to eating 10 full days of food; it's completely fair (and sane) to reason that characters to be unwilling to eat large numbers of berries.
If players insist on gluttonous characters you still have some options.
In the short term it's reasonable to apply the Poisoned condition to someone who heals 10 HP by eating 20 big Mac meals in a minute. In the long term the sheer caloric intake of healing through Goodberries would wreck the bodies of even the most cardio intensive classes.
Unfortunately, 5e doesn't provide any guidance for the DM in how to handle extreme magically induced weight gain but here are some starting points.
On another note, RP reactions may turn up because a player with so much mass would actually be quite useful in negotiating with smaller (or hungry) creatures.