The simple answer, no one has any idea but that doesn’t mean you can just drink anything. Going out of date is a difficult concept with alcohol, they certainly can but many old beers are still completely drinkable while younger ones you wouldn’t want to touch. Generally, we advise to err on the side of caution.
Defining off with any alcohol is difficult, many can be kept for years and some are even required to be, so that they reach their potential. It is partly why alcohol grew in popularity, the other reason being pretty obvious. For it to go off the contents have to break down or form something new that we don’t wish to consume. But how long or if this will happen can differ wildly between recipes. It is more likely this is down to an infection from improper cleaning than the actual homebrew going off. Outside that it can really just be down to personal preference, one flavour becoming too strong may ruin a drink for some people but not make it dangerous. So, while there is no simple timer on when a homebrew goes off, they can reach a state where you want to avoid consuming them, but how do you tell?
The easiest way to tell if your homebrew has gone off is the smell, if it is really bad you may get a sulphur smell (like rotten eggs) or something like wet paper. It is hard to describe but you will quickly recognise it, as with the taste. A harsh, sharp bitterness is one of the more common off flavours as is a yeasty or bready flavour (though this may be down to the yeast not settling yet). You should be familiar with your own homebrews and what they smell and taste like usually. A change doesn’t mean they have gone bad but if you find an old bottle be cautious at first.
How dangerous is old beer? It also really depends, as a general rule if it doesn’t smell or taste right you should avoid it. But that doesn’t necessarily make it dangerous, seeming off could just be down to a harmless reaction but you really don’t want to drink that anyway. At the same time a normal pint can give you a bad stomach anyway it is a pretty messy subject. If exposure to the air has caused the change then you don’t want to drink it as the risk of something getting in is high, though again it is unlikely to make you ill.
In the end though we advise to leave any beer which is labelled out of date or any homebrew that seems a bit weird.