Brownies are notoriously difficult to time. Unlike cakes and most cookies, you can't simply judge by looking at them, since the batter is far too dark to allow for the tell-tale browning. The tests that are usually applied to tell when cakes are done - namely, the toothpick test and gently pressing the top to see if it "springs back - don't work because brownies are often wet and fudgy when done; a brownie that is dry when it comes out of the oven is overcooked. But recipes and mixes give a range of times - which time is correct?
One strategy is to use Real Simple's tips and gently press the very edge of the brownies to check for firmness. They also suggest that the batch is done when the top looks set and dry. Try keeping a list of the times you try, so you know that when you use Brand X, 25 minutes is the best time. Another strategy is to pick a really reliable recipe and stick with it, that way you won't have to worry about the time and you're assured to try great results.