Most c-sections now use a low horizontal incision, also called a bikini cut. This type of cut normally heals within three months. Yet, the longer your scar has had time to heal, the stronger it will become, and the smallest the chance of complications will be.
There is a small risk of uterine rupture during pregnancies after a previous c-section, and especially during a vaginal birth after cesarean section. The risk is impacted by many factors, including the type of incision (horizontal, vertical or classical, or inverted T), the type of sutures following the cesarean, and... the amount of time that has passed since the c-section was carried out. It is because of this small but real risk that obstetricians are concerned about the timing.
Giving yourself the opportunity to fully recover from your c-section, and letting your body heal fully, is the best approach. If you are hoping to get pregnant relatively soon after a cesarean, because of the age gap between children or your age, for instance, it is advisable to consult with your OB before deciding to try for a baby.
If you are currently pregnant, already know you will have a c-section, and are planning a subsequent pregnancy as soon as reasonably possible, discuss this with your OB as well. Double suturing makes the scar stronger, for instance, perhaps making it possible to start trying for a baby sooner. The timing between your pregnancies may affect the type of birth you have with a later baby, making a repeat c-section more likely. You might also be interested in reading about exercises for c section recovery.