I don't think your idea is "gross" at all - you have a question that comes from curiosity and it is terrific to be curious!
Do you already have pet snakes? Or have friends that have snakes? If so, you probably have watched them eat. And if your snake is like one my son had - you can go to the pet store and purchase mice that are already dead and frozen. You thaw the mice, and then feed them to the snake. I always did feel a little sorry for the mice though, and maybe that is why you are asking if your idea is "gross"? We do need to be responsible and think carefully about what we do - being respectful of all life. So actually, I am glad you are thinking about these things. But, if you already have snakes and use the mice that are from the pet store for the purpose of feeding the snakes, I think you should feel okay about testing your idea. You can talk to your parents and your teacher, too, to see what they think and get their opinions.
Now, let's think about your idea like a real science experiment.First, what do you already know about how snakes eat? Have you watched them eat? Seen a show on TV? Have you ever noticed anything special about their jaws? Do they breathe while they eat?
Second, let's think about your "hypothesis" - that is, what kind of predictive statement can you make about your idea? Maybe something like: Species A will eat faster than species B.Or maybe: The bigger the snake, the faster they can swallow the mouse.
Third, your actual test: The only thing that should vary is the actual snakes. Everything else should be as "equal" as possible. For example, the mice should be the same size and thawed exactly the same. The time since the last meal should be the same for every snake. The temperature, lighting, and other "environmental factors" should be as equal as possible. Then, you have to be sure to time "eating" accurately. Will you start your timer once the snake has "grasped' the prey? Or from the time of the first swallow? And when is the eating over? When the mouse tail is completely in? When the snake "gulps" and resets its jaws? And then you have to time all of this exactly the same way for each snake. Finally, think about whether you have to repeat the experiment. If you compare 3 snakes, you will get three different times. But if you repeat the experiment (with the same snakes) two weeks later, will it be the same? How many times might you have to repeat the experiment to determine if there really is a nonrandom difference?
You can also start to think about the "how" and "why" parts of your question. When snakes are eating, they are pretty easy prey for other animals (like foxes or hawks). Maybe some species are really fast eaters so as to minimize this "weak period." Maybe other snakes are "slow" with eating mice because in the wild, they actually eat other kinds of food (beetles, frogs?) and so they are not great at eating mice. Or maybe it is just a size thing - the bigger the snake, the faster it can eat.
It will be a lot of work, but a LOT of fun! Good luck - and keep asking questions!!!