There are a lot of responses here that boil down to: “Just use your intuition.” This is a legitimate way to estimate, however, like any estimation technique, it

There are a lot of responses here that boil down to: “Just use your intuition.” This is a legitimate way to estimate, however, like any estimation technique, it has weaknesses that make it inappropriate for many situations. What’s most helpful is to abandon the idea that there’s “one true way” of estimating and instead educate yourself about the many estimation techniques that exist, and when and where to use them. Also keep in mind that estimates are inherently political, that they can be used to guide or to destroy, and that may be the dominant factor in the choice of technique to use. For a very good introduction to all of these factors (and others), and a survey of numerous estimation techniques, I recommend reading “Software Estimation: Demystifying the Black Art” by Steve McConnell.