GMAT Verbal Section | Re: Run On sentence ?

Hi,
I'm somewhat confused about run-onsentence.
He is a good student, he studies all day long . ......> run-on sentence, isn't it?
But, my question is:
if there is no comma or keeps other punctuation between 2 sentences, will these sentences be considered as run-on sentence ?
Here, are some examples givenbelow.
He is a good student he studies all day long ....> is it run-on sentence?
He is a good student - he studies all day long ....>is it run-on sentence?
Thanks expert...
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