Unfortunately there is no good answer to this question, it's like asking "how long is a piece of string?" It's as long as it is.
At Kellerher International we have clients that go on one date and are immediately exclusive and headed for the altar. We have others that take months to make the right connection. I usually get this question from singles who believe it's taking too long and are looking for some reassurance that "someday" is out there.
The first step I take with someone who is growing frustrated is re-ground them in the values they are searching for: have they really looked into their own heart to determine what is important for a lifetime of happiness. I then ask them to think about the last couple of dates they have had -- are the values they just shared with me in line with the reasons they liked/rejected their last opportunity. If not (e.g., if family values is a top criteria, but they rejected the last guy for his political views), then I challenge them to refresh their values. Then I ask them if the amount of effort they are putting out is in line with the importance of finding "the One". Looking for love isn't something you fit in and around the rest of your schedule, but rather it is the thing that other events get moved and canceled for. Have you been available the past four Friday and Saturday nights, or have you been skiing/playing poker/at the ball game?
There is no satisfying answer to "how long", but if you are well-rooted in the important values in a partner and are prioritizing the pursuit of "the One", than the answer to the question is "sooner" than if you weren't doing these things.