must we use "how long" or how much" in this sentence? : "how much" has she been playing tennis?

Answer 1

"How long" is more appropriate. It refers to a duration of time, so you'll receive an answer like, "for the past two months" or "since 2000."

If you're not interested in the duration, but rather the frequency, phrase the question like this, "How often has she been playing tennis?" In this case, possible answers would be, "every day" or "once a month".

Answer 2

I would use how long since it is a duration of time. If it would have been an amount or quantity it would be how much.

Answer 3

How long, because you don't buy time.