It varies but each ink tank holds 100ml of ink usually, so they outlast many sets of cartridges and you can buy ink to top them up, but at the price they are its only the cost of a couple of sets of cartridges, so well worth the initial cost.
I had one on my Epson D92 for several years, and now have one on my SX235W.
Sorry about that - it should have said "Will they definitely be recognised by my printer" - and how do they connect to the printer - do you need a lot of room around the printer for them - and do you have to have the printer lid open to use them?
I have an all in one printer/copier/scanner so I gather this won't work on mine.
I havve used Internet Ink compatibles for many years with no problems whatsoever. Also they give 10% discount for future orders. Cartridges for my Brother printer have twice as much ink as the Brother originals.