Not far. At least that is what a lot of their fans think. Even though Valve has never indicated that they are working or is planning to work on a new Portal game, the fans believe that there is going to be one and that the announcement is actually not that far away.
Most people believe that it is going to happen mainly because of the mysterious ending in Portal 2 that suggest that there might be more to the story but it could just be Valve making sure they leave something behind so that they can continue with it if they wanted to. That does not necessarily mean that they will be working on one in the near future.
Fans are saying that Valve would be making a Portal 3 announcement before the year ends but we think that is a little too optimistic since Valve has a habit of not offering third installments of their games. Half-Life 2, Left 4 Dead 2 are all great examples.