HI. I made the keifr berry soda in Fallon's new book Eat Fat Lose Fat. It says to ferment 48 hours, strain, bottle, then sit out for 48 hours then put in fridge. This one was fizzy!! (i used the powder). But most of the time my other concoctions with the live grains don't produce fizz. Pressure builds up, but once you open it it isn't really fizzy ( I am looking for a soda replacement!!) Sometimes I get a fairly fizzy one, but not sure what I do different. As for pressure, I've left mine for days (with live grains though) and nothing ever exploded. I use mason jars and also some 3 quart jars with wire and rubber locking lids I found at local grocery store.
HOpe this helps a little.