Official guidance is obviously going to be “don't do it”. Even if the airlines know it's happening or perhaps even welcomes it (Ryanair?), they probably want to avoid any sort of suggestion that they condone it, lest it become a liability.
There are a few things that might make a bit of difference (how big or efficient the airport is, whether you need to collect hold luggage and recheck it within the baggage drop-in time window or can proceed directly to the gate with an electronic boarding pass) but the main problem is that you need to plan a buffer if anything goes wrong.
As a rule of thumb, I would therefore say that something like 3 hours is very tight (I have managed it, in my younger days, with Ryanair at Stansted and it was stressful) and 5-6 hours should protect you against regular delays (lost slots, etc.) and long lines at just about any connecting airport. Beyond that, in case of flight cancellations, let alone volcanic ashes or major weather events, all bets are off.
Note that if your airline of choice only has one or two flights a day between your departure point and the connecting airport, any issue can end up being catastrophic. If the plane isn't available or the airline is forced to use a smaller one, stranded passengers will be offered some assistance (in the EU, legal requirement is a hotel night and a few hundred euros) but the delay can very easily be a day or more and there is no way you will make your connecting flight.