How much calcium propionate should I use as a bread preservative?
Most sources recommend 0.1% or so, but not more than than 0.5%. Try to use only enough to achieve the preservative results you desire. 

In other words, for 1000g of flour, start with 1g of calcium propionate. If that works good for you, then stop there. If you still think it's spoiling too soon, then next time try 2g of calcium propionate for 1000g of flour.

Adding more calcium propionate may or may mot make it last any longer, so don't use more than needed — and never more than 5g for 1000 of flour.