As an experienced pet sitter I have often run into many pet parents who free feed their dogs and cats. What do I mean by free feed? I mean putting down a bowl of food for the day and letting the pet eat as much as he wants. It usually involves refilling the bowl as it gets empty.
If you are one of these pet parents you may be asking yourself so what's wrong with that? In many cases, nothing. If your pet is an ideal weight, and just eats when he is hungry and is in good health, free feeding is generally not a problem. I do however recommend that 1) you are feeding the best quality food you can afford and 2) you do not refill the bowl without washing it out first. Do not put fresh food on top of stale food that hasn't been eaten for a day. Remember that as your dog or cat eats, some of the food falls out of their mouth and ends up back in the bowl. It is now mixed with saliva and the beginning of bacteria growth can start to happen in the remaining food in the bowl. If there is still food at the bottom of the bowl at the end of the day, throw it out, wash out the bowl and give them fresh food.
My personal belief is that dogs and cats should be fed a certain amount of food based on their size, their weight, their age, their activity level and their general health. Feeding twice a day is most common but I generally split my cats' food for the day into 4 different servings. I do this because otherwise, they would eat it all at once and be looking for more food throughout the day and because in the wild animals generally do not eat their whole "kill" at one time. Also, I can monitor how much they are actually eating and it is easy to determine if one of my kitties has lost his appetite and may not be well.
Since more than 50% of cats and dogs in the US are obese, I think it is our responsibility as pet parents to monitor their eating and make sure they stay at a healthy weight.
By the way, I started using a feeding ball for my cats. It is a plastic ball that has holes in it where you can adjust the openings which regulates how much food comes out. It is great because it gives them a chance to get a "treat" as they roll the ball around. It is challenging for them as well as exercise as they try to get food to be released.