One is that we often forget to do things if we don't make them a routine. Another is that missionaries generally receive their support checks monthly. Many others give gifts as they can, sometimes just once. Some give quarterly. Most people make a long-term commitment, but not all do.
Some people give for a year.
Some give for the duration of a project, or circumstance. Also, some donors find that their circumstances change, and they can no longer give, or give as much or as often. Donations to the ministries of New Tribes missionaries are tax-deductible. Donors will receive a receipt each time they give. To meet audit and tax requirements, ultimate use and allocation of gifts must be under the control of New Tribes Mission.