The secret to buying a life insurance policy with the right term is figuring out how long you need to be insured. You start by estimating when your children will be out on their own and no longer in need of your financial support.
So if your children are 3 and 5 now, you'd probably want a life insurance policy that covers you at least until the youngest is 22, so that's about a 20-year term. But this depends somewhat on your age as well.
Say you also want to cover your spouse for your lost income until what would be your normal retirement age, 65, and you're only 35 now. Then you would want a 30-year life insurance policy.
Keep in mind that insurance gets very expensive as you leave your 50s. So you may pay more to cover yourself until 65, even if you lock in a level-premium, 30-year policy when you are 35. Coverage past age 70 or so may be unattainable.
Life insurance is not a substitute for a retirement plan. You want to plan so that you'll have enough to live on when you retire, and you won't have to keep paying insurance premiums.
There are exceptions, however. People who start families late in life, or who have complex estate-planning issues, may well have a need for life insurance beyond the customary retirement age.