Is this a single sentence. I know I can break this into multiple sentences, but I would like to phrase this in a single, non-run-on sentence.
"Sometimes, I enjoy cooking: Whenever guests come over, I always spend hours prepping in the kitchen, and I often lose track of the time I spend cooking."
I'd say no!
You have two things to say. The first is a fact (sometimes you enjoy cooking), the second is an illustration of this fact. The fact can either precede or follow the illustration. However, running them into one, for me, makes things less clear.
"Sometimes I enjoy cooking; whenever guests come over, I always spend hours prepping in the kitchen. I often lose track of the time I spend cooking." I realize this is two sentences, and you wanted one. In a compound sentence, you don't want to combine more than two, or you will end up with a run-on. You are attempting to combine these three sentences: (1)I enjoy cooking. (2)I spend hours prepping. (3)I lose track of time.
Yes, it's grammatically OK, though a little clunky. You can use a colon between two clauses if the second one explains, summarizes, or gives an example of the first.
Spending ours prepping and losing track of time are two examples that illustrate that you really do enjoy cooking.
(By the way, that capital letter on "Whenever" needs to be made lower case.)
Nonetheless, the others are right that this is not the easiest way to do it. There's no reason not to separate them into sentences, and it will make it clearer:
I enjoy cooking. When guests come over, I spend hours prepping in the kitchen. I often lose track of the time I spend cooking.
