We used to do dry food out 24/7, but one cat got FAT, and then we did lots of research on just how unhealthy dry food is for cats. It's usually loaded with carbs and other things not natural to a feline diet.
Plus, neither of them would drink enough water. Water dish changed frequently, ever-fresh cat fountain, made no difference.
We wanted to switch to a raw diet, but neither kitty took to it (too old to change their ways, apparently... some take to it, some don't). So we switched to canned-only, and grain-free only. High protein, all meat or with some veggies (they do eat grass after all), but no grains.
The difference has been very noticeable. The fat cat lost all her weight and is healthy and active again. Their fur is healthy and shiny... Yes, it costs more than grocery-store dry food. But it's worth it for their health.

Just like I wouldn't feed my own children... say... Froot Loops, left out for them to graze on all day. Fortified with vitamins that they get all they NEED, so long as they have some milk for protein as well. I think it's a similar kind of thing...
Anyway, we usually feed them twice a day. About one can per day, so one-half can per cat. The feeding guidelines are on the can.
Here's more food for thought... some foods are more nutritionally complete than others. When we run out of the 'good stuff' and have to run across the street to the corner store to get the regular Purina or whatever to tide them over until we remember to take our stupid selves to the pet food store... they eat MORE. They are hungry and whining all day long, and will eat a full can each instead of a half can.
Even among the good brands we use, some will necessitate a half can each, others more like 2/3 of a can. It was a real revelation when we saw such a noticeable difference with their satiation according to which food they were eating!
Oh, and we also mix their servings with even more water... Like I said, they don't drink enough. Our male kitty got crystals a couple years ago, so we have to be extra-sure he gets enough moisture, that's when we started doing this. Basically they get SOUP for their meals lol... they lap it up eagerly. And it's about as far removed from dry food as you can possibly get lol...