Ruling 2260
The period of the tree tending contract must be known, and the length of the period must be such that it is possible to harvest the crop in that time. If the start of the period is specified and the end of the period is fixed as the time of harvest, it is valid.
-Ayatullah Sistani, Practical Laws of Islam, Tree Tending Contract (Musaqah) and Tree Planting Contract (Mugharasah)