Depending on Juice, for flavors I care about I get about 2ml to 3ml then I have to toss the carto away. I mainly use Boge in Ibtanked. I know it's on the low side, but I notice juice flavor and taste dropped to about 70% to 80% comparing to a newly filled carto after about 2 to 3ml. It's still completely vapeable with 80% flavor and the draw is still perfect and not stiffen up or anything, but I choose to put a new one in to get 100% flavor back.
And for flavors I don't use everyday, I can tolerate when flavor drops to as much as about 30~40% before I toss it, so I get about 10~15ml before I throw them away.