Lessons get unlocked with any radical or kanji you level up to guru. But not with vocab. All items are on an individual timeline (though they tend to group according to how you group your lessons, as long as you get them all correct).
I usually check my dashboard where I have the script [Userscript] Dashboard Progress Plus installed. But I think ou can see it on the vanilla dashboard as well. I look at how far along my kanji and radicals are, and when the next review of the furthest along items is. And when I will likely unlock new lessons.