Let's take this utterance apart. This って functions like a topic marker. See this discussion
Difference between って and は as topic marker
It's put in the form of a question, so 「じゃない？」 means "isn't it?" And the statement that the じゃない？ seeks confirmation of is 「食べてる時間って暇」 ≈ 「食べてる時間は暇」: "the time when you eat is free" or "[someone is] free during the time of eating"
So put together, this utterance means "You are free when you eat, isn't it?" The implication is: when you eat, it's your free time, and you can watch TV, talk to your friend on the phone, relax, daydream, listen to the music, etc. I am using a generic "you" here. The speaker may intend this as a question to all the listeners, or more likely, saying this as a general observation.
Note: 暇 in Japanese can also imply being bored and personally I think the line between "bored" and "free" is blurry. That's why when you want to talk to your boss during work, you should never ask if they are 暇. That would imply you think they are bored and have nothing important to do.
暇だ！ = I'm bored!
休みだから暇だよ = I'm bored because it's a day off.
With that in mind, also per @aguijonazo's comment, this could also be a comment about how the time spent eating seems boring or wasteful.