It's a hard question to answer really since we don't really know what the final goal will be. In the end we don't have deadlines or bottom line or stockholders that we have to answer to. This puts us in a unique ( some would say good, some would say bad ) position. Normally after every version we sort of plan out what we want to do for the next big release and then work to that.
To me I can see no end in site for the project as a whole, as new concepts are developed they are added into the game and it will always be like that. I can't imagine a point where I log into IRC and see "Ok, PlaneShift is offical done" since there is always changes that can be made or improvements.
Think of it like this, can you ask a stamp collector when he's finished with his hobby? Or is a painter finally finished painting?
There are still large areas of the game we have not explored yet such as ranged weapons, player built housing, mounts, siege weapons etc ( Not saying that those will be added any time soon but those ideas are out there ).