A well known DIY fertellizer with a horrible smell :p I've made my first 5 liters few months ago. I wonder, how long does this stuff actually last? I've kept in a closed milk carton (think that's what they use it for in the US) ever since.

Does it go 'bad'? (though it couldn't possibly get any worse than this )

I did once see an English tv gardener make up some comfrey liquid. Found it -
http://www.gardenersworld.com/...

I know how it's made, I just want to know if this also has a shelf life? Is there any time after which it's adviced to not use it anymore and just make a new batch?

I haven't run across any real shelf life information. Some folks keep it for the entire growing season or a year. Some only make small batches to use quickly because of the smell. Others question the anaerobic nature of the stuff. One thing I did read is that it's best kept in a cool, dark place.

If the sole virtue of that "tea" is that it returns nutrients to the soil, you can eliminate the smelly middle-man.

Just chop the leaves and compost them instead of brewing tea.

Or chop them up and use them as mulch.

Or just use them as mulch.

The minerals including NPK will still get into the soil, just more slowly and less stinky. (Less stinkily?)

If the virtue of that tea is that someone thinks anaerobic microbes or anaerobic fermentation products are GOOD for soil or plants ... that's their theory and they can speak in favor of bad smells and whether "fresh bad smells" are better in their theory than "old bad smells".

I just barely believe that aerobic compost tea is a plausible theory: that encouraging aerobic COMPOST microbes to grow on leaf surfaces and replenish microbe populations in the soil might be better for plants than just giving them the compost itself, straight up, on the soil.