I use this with my Series 7 razor. The razor came with one cartridge, and I used it daily and it was gone after about 45 days. I came to realize that it is not necessary to clean the razor daily.
My new process is that I remove the cutter head after each shave, shake out any hair, then rinse. I leave the two parts separate to dry. Once a week, I take the head off, shake the hair out, but do not rinse the razor. On this day, I put the razor back together again, and place in the cleaning/charging station to run through the cycle.
Using this process, to minimize the hair pieces that get into the fluid, and to reduce the product consumption should allow each cartridge to last a very long time.
The razor does come with a hygiene indicator, but as far as I can tell, it is just an indicator of how many times the razor has been used since running through the cleaner. I feel that my interim cleaning processes is adequate to keep the razor clean.
If you are thinking, why use the solution at all, I think it is because that added benefit of cleaning and lubricating the cutter head.