Time Limits

We are use to time limits in a world that is run by a clock. Seventy-five minutes to take a test, seven minutes to get to your class, a week to do a project, a month to plan that party. We have dates and places to be at certain points. We even have time limits in life. When you should get married, when you should have a baby, how long you should wait before you meet the parents, when you apply to school and graduate and etc.
Tonight one of my conversations revolved around the right time to say I love you, and while we both agree it should be when you feel it, we were puzzled by what should happen if they don't say it back for a while. Now I've never told someone I loved them first, so I'm not sure what it is like not to hear it back, but I'm sure it is awful and this is one of the biggest reasons why. My worry is that if someone says it first and the other doesn't say it back how long do you hold on to the relationship? Obviously you wouldn't just leave right then, but do you wait 3 months, half a year, forever? When do you realize that he isn't ever going to love you, that this relationship isn't going anywhere and you need to pick up your dignity and move on?
By saying it first I worry it is like putting a time limit on your relationship.
Saying "I love you" is like setting an alarm in your relationship.