It varys, on the whole it takes only a few seconds but there are times when it takes up to 20-30 which can be frustrating.
As I said on the whole its good.
It plays the song wirelessly but also creates its own buffer as the song is being played.
Ie 20 seconds of play time it might have a minutes worth of the song stored, 40 seconds 2 minutes etc.
They've done this so if you do lose connection With the device it won't just stop playing.
Hope this helps