I wouldn't eat more than a can a day. Most tuna has high amounts of mercury which can be harmful to your body, which negates the positive aspects of tuna (high protein, omega 3). I, personally, eat half a can of day... there are many other sources of protein out there - balance is key.
You can get protein from other sources such as chicken and eggs, and you can get Omega 3 from eating flaxseed. The last thing you want is to be eating so much tuna and later getting bored of it (it does happen!) For example: Every one of us has had that "favourite song" which we used to play over and over again... till we got bored of it... and than we never listened to it again. I know it's a totally different analogy, but it's just something to consider.
@ Davezz - Too much quicksilver? lol. It's called mercury. Quicksilver is the name of a clothing/skating company, haha.