Heating time of an aluminum plate
Your calculations about the current and the power of your resistors are okay.

To know how much time it'll take to heat up your plate with those resistors to a certain temperature, you would need other data (most of which will not be available until you build this thing and start measurements). The time depends mostly on the initial temperature (the ambient temperatute), and on the ability of your plate to dissipate the heat (to transfer the energy to the surrounding air): the better it convects the heat, the longer it will take to heat up.