Your average MP3 is about 7 MB. There are 2560MB in 2.5GB. Since the size of the music files will vary slightly, you can expect to get anywhere from 320-375 songs. Just check up on the amount of data that you have used every couple of days to make sure that you don't go over!
You can disable apps/services running in the background so they won't waste your data for no reason. That being said, you're going to be able to download 350-400 songs for 2.5GB. Depends on the kb ratio though.