How long is a piece of rope?
Since no two printers, images or profiles are exactly the same, ink coverage on the print will vary. A light colored image will use less ink than a dark image.
If you're just starting out, I would estimate 500ml of ink will cover about 500 sq ft of media for estimating purposes. As you produce more product you'll get a better idea of what your average use is.
Also, when I was printing in house, I used Onyx Postershop which showed you how much ink was used. I would assume most other rip software would have a similar feature. You may have to dig for the info though.
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