Please tell me how to calculate how long this speaker can play at maximum volume?

Theoretical calculation is relatively simple, the use of the length of time = capacity / power, pay attention to the unity of the unit on the line. According to what you said is 15*6/100, the result is 0.9 hours. This is the theoretical value calculated according to the data you provided. But in practice, very few speakers will run at their rated power, even if you turn them up to max volume. So why would he say that the battery discharge is only 15V2A, which means that the discharge power is only 30W. so in practical terms it is closer to the actual power of 30W. 15*6/30=3, which is 3 hours.