I don't normally do their quest but as far as I know, there is a time limit once Emily tells you that Tisha is missing and you can fail to find her if you take too long. This was a few versions back when I did it, but I don't think it's changed.
Viewing post in Tips and tricks thread... (THE REVIVAL)
Weird... I remember playing 0.5.16a build and waited for what must have been 30-60 days before I went to find Tisha and still managed to get her with no problem. Although I deleted my save so I might recall the details wrong and just got lucky within the cut off date. Thank you very much for the heads up.