You wouldn't keep old milk in your fridge for longer than a month, so why let the time curdle on an "I love you"? Whether she told you in a fit of passion or you dropped the "L" bomb on a weekend getaway, there is an intended time frame that you must adhere to before the expiration date is up to say it back or forever hold your peace.

If she uttered it: If you like her but don't love her, you should probably let her know. You can get away with just never saying it, but after a couple weeks she will start wondering if you are ever going to say it and might try to find ways to get you to spit it out. It is best to just let her know you are not on the same level with your feelings yet, and she will hopefully leave it alone. If you do feel the same way, but are being hesitant, try not to let more than two weeks pass by or she might start looking for someone who will return the feelings.

If you uttered it: If she did not immediately proclaim her love for you within seconds, it does not mean that all hope is lost. She might just be unsure of her feelings, or does not want to rush into things too quickly. Give her a couple days and wait for her to bring it up (be assured, she IS thinking about it--a lot). After a week, the curdling has begun and once a couple weeks pass by, it might mean that she simply does not feel the same way. Once it has hit a month, bring it up in light conversation and see what she says.