The number of years of the fruits - i. e. according to the number of harvests. The average value of a yearly crop might of course be estimated, and the sabbatical years were to be deducted from the series.
Related Commentaries of Leviticus 25:15
Leviticus 25:15-16
15 According to the number of years after the jubile thou shalt buy of thy neighbour, and according unto the number of years of the fruits he shall sell unto thee:
16 According to the multitude of years thou shalt increase the price thereof, and according to the fewness of years thou shalt diminish the price of it: for according to the number of the years of the fruits doth he sell unto thee.