This is a function of latitude and season. Apart from that, it depends on local landscape, weather, and your own eyes. On a rainy day in a dense forest, it may already be quite dark at sunset. On a clear day on a treeless plain, there's still plenty of daylight at sunset for most people, even though it's a lot darker than at midday.
Civil twilight is defined as the period between sunrise and when it gets too dark to do everyday outside activities. That typically corresponds to the Sun being around 6° below the horizon. Depending on latitude, that may be as short as 20 minutes (equator) or as long as several weeks (poles). Near the Arctic Circle around the equinoxes, it tends to be around an hour.
You said you preferred not to use sunset tables, but it really is the best way to go. I tend to use the US Navy calculator. For your question, you only need to pay attention to the difference between sunset and "end of civil twilight". In an open area, that is a good proxy for the period during which you won't need artificial light for outdoor activities. When the weather is very poor and you're in a forest or deep valley, you'll need to reduce this period a bit.