Kratom is pretty fast-acting and it seems to last around 6 hours or so at best, with the peak of effects lasting in the 4 hour range, and near 8 hrs you might have a few lingering effects... Because it kicks in so fast, 15 minutes for me but depending on the strain, sometimes up to a half hour....I'll redose fairly rapidly if the desired effects aren't achieved...like within an hour, because if you wait too long, you'll never get the desired effect because your initial dose is fading while you're working on adding to it 6 hours later....
For me, 6-8 grams tends to be about the sweet spot with Maeng Da. Dosing higher than that gives me diminishing effects, and it starts to become a downer. And since Maeng Da isn't good for pain, I find it defeats the purpose to dose higher. If I'm going for a big dose, I'll go with Bali because you wind up with a calming, pain-killing, euphoric effect. Maeng Da is more of a...work your butt off, get things done and have fun at it type of buzz.
Funny that you say Kratom kicks in in 15 mins for you because for me it takes at least 40 mins before I feel any effects at all and usually about an hour before I feel peak effects.