It's an utter crapshoot. Whatever your level is, you can improve a lot. Unless your goal is 5d, it's probably possible. Unless your goal is 18k, it's not guaranteed. It's hard not to think about the rank you'd like to be (Five years in, I still imagine that this year I'll finally break 2k kgs), but it's a distraction.
If you set a goal, it will be a distraction. Either it will tell you "I have done enough" when you could improve faster, or it will be an unrealistic goal that tells you that you've failed while you're still improving.
As far as things that calm my worries about improvement, I've always liked something Dieter (of senseis' library) wrote:
To speak of myself, I never played server Go until one year ago. Hence, my rank progress has been rather linear. I became 12k after one year, and progressively 9k, 6k, 4k, 3k, 2k, 1k, 1d, 2d. The last five years I have been improving one grade a year. I would have thought for someone to be 9k after two years it would be rather impossible to become a dan player, if they don't change their way or amount of playing and learning. But to my surprise and relief, it isn't.
