The short and sweet answer is three years. Churches started following this three year model around 50 years ago or so. Time has proven that in a congregational setting four years seems to be too long and two years is not quite long enough.
However, you do see institutions like schools operate on a longer time frame sometimes. The mega-gifts that are the backbone of a multi-million dollar campaign can take longer to come in. If you are the development officer at a private college, you are not going to quibble if the person who is contributing $10 million needs five years to fulfill this pledge. You say thank you and try to get the name on a pledge card as quickly as possible.
Likewise, we have conducted congregational campaigns with a two year giving period and had success. The factors that contributed to this were the amount needed to be raised and our knowledge about key donors. We knew the majority of the lead gifts would not be affected by the time frame; the largest givers did not need the extra year to fulfill their pledge.
But for most non-profits and churches, three years is the sweet spot--enough time to gather large gifts but short enough to prevent donor fatigue.