How do you decide how much of your portfolio should be in bonds versus stocks?

Many beginners pick a split once and never think about it again, while others change it every time the news shifts. I've found it helpful to start with the question of time horizon: money you need in two years usually calls for a very different mix than money you won't touch for twenty. Some people anchor on a rule of thumb, such as holding a percentage in bonds that matches their age, while others start from how much a drop in value would make them sell in a panic. Both approaches can work, but they answer slightly different questions, and it's worth knowing which one you're really using. What's the main factor that shaped your own stock-and-bond split: your timeline, your comfort with swings, or something else?