How can a Pure Titans recover the full body after extended period?
There is no specification on the manga or anime about that. But it seems that any "pure" titan as long it gets the spinal fluid of 1/9 titans is able to go back to being a human no matter how long it takes then in their pure titan form. Yet again, all of Ymir's people seem to see the invisible roads or at least the people that turn into one of the  1/9 titans. Time pass different in that place, seems to take years or thousands of years[forever]. Ymir, just mentioned that it was like a never ending nightmare to be in the pure titan form.  

Another way of putting it is, all pure titans are female eggs, and the 1/9 are the sperm, when of of the 1/9 titan gets eaten by an egg, it becomes a human, no matter how long it was in that place.