User blog comment:Omega natsu2/Chapter 385: Spirit King vs. Demon King, Predictions./@comment-77.194.131.143-20140523065533/@comment-7418318-20140523160756

He could have given Lucy a set amount of magic power and once it runs out, it's gone. Or he could have given her access to Aquaruis' powers for a limited amount of uses/times like Mavis did with Fairy Glitter.

Personally I hope she'll be able to keep it at least until Aquarius' key is restored (if it is ever restored), but with the little to no information we have right now and the different ways MS and MP translated it, we'll just have to wait and see to know for sure.