Capacity planning for space

We have big ovens in which we have to dry items for  a number of hours.  I have a fix routing time for this as it doesn't make a difference if I have one item or 500, it will take 35 hours to dry.  I need to be able to manage the capacity of my oven obviously with the hours I'm using it but also with the space the items I'm putting in take.  All items have the same unit of measure.  Any suggestions on how I can do this?