I just got my Mini a couple days ago and so far my batteries are lasting around 8 hours of moderate (not heavy) vaping. I'm using the AW IMR 18350 (700 mAh) battery with a Boge LR 2.0 carto. I'm very surprised by this because those same batteries only last me 3 or 4 hours on my Mechanical Mod. With the Mini, they're powerful and long lasting, which is of course just fine with me.

If you don't have a Mini yet, when you get it, just click the button 11 times and it'll do a battery check for you. I've "heard" that the hi reading could be as much as 4.4 or so, going down to about 3.3 or 3.4 before the button starts to blink and tell you to charge the battery. Anyhow, if you're concerned about the batteries and how long they'll last, I think for such small batteries they last a long time. I would take an extra battery with me if I were going out for the day but probably wouldn't have to use it.
Hopefully someone who uses a 1.5 dual coil carto will jump in here and give you more information.