John Gill Commentary Leviticus 26:35

"As long as it lieth desolate it shall have rest, even the rest which it had not in your sabbaths, when ye dwelt upon it." — Leviticus 26:35 (ASV)

As long as it lies desolate it shall rest From tillage, neither man nor beast working upon it; for which reason such a space of time was called a sabbath: because it did not rest in your sabbaths, when you dwelt upon it ;

they manured and tilled it on the seventh years, as on others, neglecting the command which God had given them; and this they did not once or twice, but many times, while they were dwellers in the land; which seems to confirm pretty much the notion of their having omitted so many years, though that cannot be affirmed with certainty; see Jarchi on the place.