~13 hours, more or less. I got 8, 2, 8, which is probably the best die rolls anyone has ever gotten on this boss. Guess I can stop complaining about RNG now, haha.
I really, really love this fight (that being said, I don't think I'll ever want to play it again). Pretty much everything can be dealt with once you have enough experience and understand how to set yourself up in every conceivable situation. It really feels like a boss fight, much more so than the usual random danmaku while you fire your gun. My biggest issue was the avoidance - by the end I could get to it once every five minutes on average, but god do I need to work on my apple reading and dodging :E
EDIT: I uploaded a video of the fight even though everyone's seen it a million times already: