The hissing stopped around 11 seconds and the water quiesced moments thereafter. Knowing nothing about your coffee preferences or the particular HX espresso machine other than the video above, I would try 4 seconds past the end of hissing, wait 10-15 seconds, then pull the shot. Fine tune to taste:
- Bitter, hotplate flavor - flush an additional 5 seconds, increase rebound 5 seconds; this flushes more water from the HX and compensates by allowing the grouphead a few extra seconds to stabilize.
- Mixture of sour/bitter - shorten flush by 3-5 seconds, shorten rebound by 3-5 seconds; this avoids overcooling and then overshooting temperature, i.e., the dreaded "U" brew temperature profile.
- Somewhat sour, dull flavor - shorten flush by 3-5 seconds, keep rebound the same or increase slightly if it shows previous symptoms. Note: I find that lightly roasted fruity single origin coffees often taste better with this low hump, declining brew temperature profile than chocolaty blends.
There's no science to the above, it's purely guesswork based on lots of practice. If you want to reduce the guesswork on the cheap, search the FAQs, there's plenty of previous discussion in threads like Seeking low tech way to measure brew temperature
, What's necessary to make a thermocoupled portafilter?
, and Can't tell when the HX water dance ends