Remember that, even though there are little short sentences, they are still sentences if they have their own subject and verb and can stand alone. Punctuating them with a comma creates a run-on.
...That's right. He was long overdue for the visit.
...Let's see. I think it was May.
...He's an adult. He needs to get a job.
...He works on Saturdays during the school year. He is not available.
...We mailed them out on a Monday. It was fine.
...She may have wanted more than that. I don't know.
Would these be good places for a semicolon? No. The semicolon works only when the ideas are really closely related or the construction is parallel.
...He is at home; he is not at work.
...She likes to read; he likes to fish.
Happy punctuating!
Margie