The amount of time needed to increase muscle tone or definition varies, but I usually tell my fitness clients to expect it to take about 2 months. Some muscle groups will tend to show improvements more quickly than others (the arms, for example), but results will vary overwhelmingly depending on several factors, including body type, workout regimen, and diet/nutrition.
For example, for clients who are overweight when beginning their training, I make sure they are including cardio as well as strength training. The two types of workouts will complement each other, and if the proper balance is prescribed, the correct amount of cardio will help decrease the amount of time it will take for you to see changes in muscle tone.
Maintaining a diet that is natural and rich in vitamins and minerals can also increase your body's ability to support muscle tone and strengthening. I recommend that anyone starting a goal-oriented workout routine see a nutritionist to make sure they are maximizing the results of their workouts by supporting their body's work with the proper nutrition.