If you have a goal, you can adjust your daily habits, so that they make it easier to reach your goal. Sometimes, goals can be hard, but that's not the case with daily habits, because they're seomthing you'll do uncounsiously most of the times.
Being fit, is strictl related to happiness. If you're not happy with your image, you cannot be happy.
I think you are supposed to measure your goals in steps. Which means, you can divide a goal into smaller steps, and each time you reach a "subgoal", you're closer to meating your final goal.
It depends strictly on the goal itself. If it's something big, it might take you monts, years , your whole childhood or a whole lifetime. If it's not that big or intense, it can be days, weeks, months...
You will always know that, because, even if YOU are not paying attention to it, your brain will keep track on your progress, because your brain will be trainted to feel happy whenever you ste-up. That's because we are programmed to do what makes as happy, so uncousiously we're always searching for happnes, and completng goals certainoy provides that.
Hope it helped,
BioTeacher101