A court awards alimony when one spouse needs financial assistance and the other has the ability to help. In general, the longer the marriage and the bigger the difference in earning capabilities of the spouses, the more chance for an award of alimony. In a really long marriage, the court may award permanent alimony.
How long alimony lasts depends on the length of the marriage and the relative incomes of the spouses. Also, if one spouse has been raising children and out of the work force, the court will award a longer period of alimony to allow for retraining and a return to work. Most of the time, the divorce judgment will state a specific ending date for alimony, but if it doesn't, the rule is usually that alimony ends when the recipient dies or remarries.
If you suspect your spouse is not reporting all the money that's coming in and you want more support based on what you think the real numbers are, you must prove it. You can testify as to what your spouse told you about income or have other people testify about your spouse's income. Another way of proving that income is being underreported is to prove that your marital lifestyle cost a certain amount and that was fully paid for with your spouse’s earnings during the marriage — leading to the conclusion that your spouse makes enough to support that lifestyle.
If a spouse is capable of making more than he or she is actually earning--for example, a person trained as a doctor is working as a personal trainer in a gym--then a court deciding on alimony can "impute" income to a spouse. That means it will calculate the alimony as if the spouse actually made the higher amount.