Every week (or depending on your studio, every day or every month), place a tick mark on the container showing how much is left. This barrel of oil went from being 2/3 full on April 8th to 1/3 full on May 20. 6 weeks to use one third of a barrel, so I can predict I have about 6 weeks left until I run out, around June 1.
I'll be contacting the supplier today to let them know my expected date for restocking. I can confirm pricing and lead time for restocking this item.