After the Seven Years arc The Celestial Spirits throw a party for Lucy and her friends and when they leave they discover 1 day in the spirit world equals 3 months in the real world if that is so then 1 and a half hours in the real world equals 1 minute in the Celestial world so how do Celestial Spirits have time to recover? For example in the seven years/s class arc Lucy summoned Torus to fight voodu doll guy and he gets annihilated but then she summons him to fight Hades/Prect and he was fine by my calculation he had 1 maybe 2 minutes to recover before he was back in the fight. A side note this also means that Loke was banished for a whole whopping 12 days and everyone on Tenrou was frozen in time for 28 days in spirit world time
This seems like a very minor plot hole. I say plot hole because, as long a the author doesn't explain it, it's fair to assume that he didn't realize it.
On the other hand, any future explanation could be something like "Since they are spirits in the spirit world, they regenerate very quickly any loss of health or stamina when they stay there. That kind of improvised explanation would efficiently retroactively fix everything and would no longer be a plot hole because it "sounds obvious".
The author might also already have this explanation (or another similar one), but is holding it until the time seems right, because most characters in fairy tail are very dumb for the sake of comedy, so they probably wouldn't think about such matters and nobody would be surprised they didn't. As for the few smart characters, they could think it is general knowledge and never bothered mentioning it yet.