Some factors you may need to take into account before you decide on the length of your term insurance policy are why you need life insurance in the first place. Is it to cover the cost of a mortgage? If so, then you would need to take out the policy for the term of your mortgage. You need to also keep in mind that the older you are, the higher the premiums, so going in for life insurance when you are younger will be advisable.
Really it comes down to what or who you're really trying to protect. If you have young children (or even older) then you might want to think about when you expect them to be financially independent and not rely on you any more and therefore to cover for the period up until that time.
Also, it is possible to add on another policy to cover your family's costs and funeral bills.
Finally, the length of your life term insurance policy should be determined by your needs and by your budget.