A marriage is for a lifetime. But this married lifetime is divided into many different stages; these are described briefly below.
The initial stages: the matching, the milestones and the revaluation
Once you are married, there is still plenty left to discover and realise about your partner. This is where you get adjusted into your different roles; whether you work at the office or at home. You divide your responsibilities and duties and get moulded into them. You tend to complement each other and the foundations of your marriage are set.
Milestones: this is when you complete three to five years into your marriage. At this point you know everything that there is to know about each other, the good, the bad and the ugly. You may also have developed so many different personality traits to better suit your spouses.
Revaluation: this happens when you are closing into the first decade of your marriage. If you have kids you tend to be more family inclined. Maturity sets in too. You tend to understand your relatives better and there is an element of re-assessment where you tend to re-assess each other and your families as well.
The later stages
When you enter the second and third decade of marriage, you have gone through a million moments of happiness, joy, frustration, anger, etc. You have gone through so much with each other and yet there is so much more to experience. Once your children are settled you may take time out to appreciate and cherish each other. This is when most couples renew their vows or go for a second honeymoon.
Everyone goes through a mid-life crisis phase at some point or the other. This usually happens when you hit your fifties. In these times, both partners should provide support to each other. This is happens when the realisation that old age has set, unnerves people and really shakes them. You go through a stage of self-doubt and uncertainty about what life has to offer next. In this stage, your marriage is really tested.
During the last few decades you realise that your spouse has stuck with you during all the trying times well as the good times. You realise that you will spend the rest of your days together and this fills you with an immense feeling of satisfaction and happiness. You could not have wished for a better life with a better person.