Peter is correct, it is a complex and rather illusive calculation. I've been using a 9900 for a little more that three years, but do to the fact that I've had a number of issues with the printer I'm probably not the person you want to hear from. However, I'll keep my comments to the particular question at hand. I recently had the third print head installed and the service tech, with whom I have become quite familiar, noticed that a number of carts were indeed past their "best before" date. He said that Epson had recently told them to let their warranty customers know that expired carts can eventually cause issues with their print heads on the X900 series printers. I found that rather suspicious and still don't put much faith in that statement. The tech had several new carts in his vehicle and gave them to me to replace the expired ones. I have printed with two 7600 printers for many years and have never had problems with expired carts, and I don't believe that was the cause of the print head failures on my 9900 either. I have also used the 700ml carts, especially the LK and LLK, since they are used in basically every image you print. My business has leveled out during that past year+ and I will not be ordering the larger carts any longer. That way I can turn over the carts more quickly, although I do like to have at least one backup for every colour at all times, in case of a chip failure on a new one. I have also adopted a routine of agitating the expired carts once every two weeks, which I think might help alleviate any sludge(thickening) of ink. Again, I am not totally sold on that explanation from Epson, but since I have now run out of warranty I really must take every conceivable precaution at my disposable. And speaking of warranties, the one suggestion I have for you with your new 7900 is to extend the warranty for as long as you can. Within the first year of my 9900's life the warranty service calls would have almost paid for the machine. The warranty extension is not an inexpensive proposition, but in my opinion it is a necessary insurance policy. Although I am now without that insurance I feel I have enough information ay my disposal to cope with most issues, snd this forum is of course an invaluable resource as well.
I guess I have yet again become somewhat carried away with my response, but I hope it has been useful in some way Yvan. Considering the volume you have mentioned I personally wouldn't consider the 700ml carts. The saving in cost could prove to be more costly in the long run, but of course that's just speculation on my part. If you find that you are going through the 350ml carts quickly you should certainly take inventory and let that influence your decision on this matter. Also, if you find that you have periods of a week or more without use, be sure to agitate the carts occasionally and run at least one print per day, just to keep the nozzles busy. Make sure it's an image that includes a good mix of all colours.
Good luck Yvan,