Well the iFi iDSD Micro is rated for 14hrs in "ECO" mode, 10hrs in "NORMAL" mode, and 6hrs in "TURBO" mode. And it has TONS of power, so unless you're trying to drive some heavy-duty "Home Audiophile" cans, I can't imagine anything portable needing more than "NORMAL" AND MORE LIKELY, you will be able to get by with "ECO" mode. On top of that, if your DX90's battery dies, you can charge it off of the iDSDs *Correction*4800mAh internal battery, or since the iDSD is also a REALLY nice DAC, you could substitute-out your iPhone/iPad if you have one and bypass the internal Apple DAC for the superior iDSD's DAC. (The same goes for Android OTG connectivity if you have an Android-based phone instead.)
Mine just arrived and I can't wait to finish burning it in!
Ken N.