help on formula to estimate production date

aim: to be able to create a forecast/estimate of delivery date

Let's say I have 50 cases sent for printing today (x days).

but the printer is only able to print only 13 per day. 

the first 13 cases will done (x +1 day) while the next 13 will be done at (x +2) and the next 13 done and (x +3). then at x +4 i have y number of cases to produce.

the production rate is constant but the number of cases for production varies on a daily basis.

what functions/formula do I use and also I need to include an actual date to the x +2, x+3 days?

I am new to this, pls help.