It's normally pronounced the same as Nina. However, I could see it being pronounced NAY-na too if that's how someone wanted to pronounce it. Some names that end in -ena that would be pronounced that way (AY-na) are Elena, Helena, Magdalena, Madalena, and Malena. There are a few others I saw when I searched for *ena that I think would be pronounced that way too, but wasn't sure.
**Vita Brevis, Ars Longa**