It also depends on if you are using sashing and border(s) :D:D:D
Multiply 6" (the finished size of the block) by how many across and down.
6" X 14 blocks = 84" for example
Do you want the quilt to fit just the top of the mattress? Hang over each side 10 inches? Do you want it to cover the pillows?
Sometimes it is best to actually measure the mattress, some are deeper and could even be longer than others :D:D:D