My first test got me 2ml on the 18350 using a 3.0 ohm boge at 4.8v (really hard to stick to just one flavor gotta tell ya, or just one mod to get a good runtime test too). Anyhow, got 15 hrs use on it as I wasn't chain vaping it.
Another who's testing with me got 3ml of use over 5hs of hard vaping using a 3.0 ohm xxl boge at 4.2v.
Someone else just reported getting 9hrs of use, forget what his juice level was though.
Anyhow, if you're really after longevity, you really can't go wrong with the full size and snag the 18490 or the 18650 or both. Then maybe treat yourself to a mini later as a backup and for a stealthy carry out when you know what your vape times will be.
Otherwise, you could always just carry one spare 18350 with you when you go out as backup. Darn battery is so small, about the size of a 3ml tank.
Just some ideas and hope the info helps you decide.