|Robin, Lily, Ted and Barney helped Marshall deal with the death of his father|
("Last Words" How I Met Your Mother)
2030 - Ted tells children that the story of how he met their mother; the boy, Luke appears to be thirteen years old and Leia appears to be fifteen.
2021 - Ted tells Wendy the Waitress that he has two kids and is married to a wonderful woman. ("Garbage Island").
2015 - Since the first born, Leia, seems to be fifteen in 2030, it is logical to assume she is born this year. Knowing how Ted is so ready to be a father, it is safe to assume that he and the Mother only waited a year to have kids, and were married in...
2014 - Knowing Ted and judging by his past relationships, it is safe to assume that they would be together for two years before tying the knot, which means by...
2012 - Ted and the Mother become a couple, and judging by how Ted wins women over with great romantic gestures, he will meet the Mother in...
late 2011 or early 2012.
Aside from the above argument being chronologically sound, I add another argument that Ted should meet the Mother soon, for the simple reason that... if the show is stretched beyond season seven, it will take its toll on the story. The plot will suffer because of the desire to lengthen the show, and milk it for what it's worth. As of now, critics are saying that the show has gone past its prime (season two being the best, they say), and is going farther and farther from the central plot (a father telling his kids how he met their mother). Although the show's concept (backward narration) is new and original, this is also the show's greatest limitation. While making the story and script, the writers have to take in mind:
1. What has been already said, to keep the continuity going
2. What this will all lead to. Ted meets the Mother in a manner that solidifies what has been said not only about Ted but also about his friends as well.
3. That in developing Ted's character, his friends' characters have to grow as well. That in creating a fitting ending for Ted's story they have to create an end--actual or constructive--for the other characters as well.
The season could end now, while it's on its sixth, but since the other characters' stories have to be ended as well, so as to make a 'full story', I believe it will and should end in Season 7 with Barney and Robin's wedding (or and I believe this is a remote possibility, Robin's wedding to some other guy, or Barney's wedding to some other girl).