He hasn't had any real rest since he entered Impel Down almost 24 hours ago, having been fighting guards and monsters, the harshness of each level of Impel Down, suffering from Magellan's poison, and going through Ivankov's healing hormones. He's only been as strong as he's been because of Ivankov's adrenaline booster hormones too. He's fighting against the bulk of the Marine forces, Vice Admirals, Pacifistas, Shichibukai, and Admirals. He's out of his league, his allies aren't his crew, and he keeps running forward by himself into the fray. It's amazing that he managed to get so far before clocking out, frankly.
That said, we may see an instant recovery soon, like during Arabasta when it turned out he collapsed only to wake up from a "nap" for a few minutes. Ivankov's vigor/adrenaline hormones aren't supposed to wear off and cause a crash for a full day, though it's possible Luffy's managed to "use them up" in such a short time frame.
Betting on Jinbei or another ally sacrificing themselves to an Admiral, VA, or Shichibukai in order for Ace to be freed, when Luffy wakes up, busts out an Emperor's Haki-fueled punch, and promptly collapses for a week-long sleep-n'-eat session. Everybody pulls back, total stalemate overall. Ace is saved, which is a point in WB's favor, but Whitebeard's growing weakness in old age has become apparent, weakening his influence and leading to some of his territories coming into question. WB officially "retires," handing down leadership to one of his commanders (probably Marco, I doubt he would give the responsibility to Ace so soon).