Well, in their respective opening weeks on DS, DQIV, DQV and DQVI did 600k, 680k and 900k. The LT numbers of the first two ended up roughly twice their FW, with 1.27m for IV and 1.36m for V, while DQVI only managed 50% more, ending with 1.35m.
I'd wager that the reason for DQVI's comparatively weak legs was the fact that the DS had already entered its precipitous decline by its release. Considering that the 3DS is pretty much exactly hitting its prime right now and that these shipments suggest first week sales more in line with DQVI than with the others (~800k?), 1.5m LT should be a piece of cake. In fact, if all goes well and 3DS really has that best year ever this year, even 2m could theoretically be possible, including digital sales.
Oh, this is fun! DQVII is making me happy both by slaughtering the charts and by being my second most anticipated game right now.