"Decibels" are used to measure serveral different quantities. Your link seems to be talking about "sound power level" (SPL) whcih measures the total acoustic power that the sound source (i.e. the scream) generates. That is not the same as "how loud the scream sounds" which depends how far away from the source you are, and the fact that the sound may be directional.
1 dB SPL is arbitrarily defined as ##10^{12}## watts of acoustic power, so 80 dB SPL = 0.0001 W
Assuming 80db is the right SPL, your link gives the time if you can somehow capture all the power from the scream and use it to heat the coffee.
Edit: I was writing this when Simon Birdge posted, but note that SPL is measured in watts, not in watts/meter^2. The OP's link isn't very clear exactly what it means by "decibels", though.
