http://www.newadvent.org/cathen/08534a.htm
Year of Jubilee (Hebrew)
According to the Pentateuchal legislation contained in Leviticus, a Jubilee year is the year that follows immediately seven successive Sabbatic years (the Sabbatic year being the seventh year of a seven-year cycle). Accordingly, the Jubilee year takes place at the end of seven times seven years, i.e. at the end of every forty-nine years, or the fiftieth. Hence, the institution of the Jubilee-year system is but an extension or the working out of the Sabbatic-year legislation, viz. that as at the end of every six years there succeeds a Sabbatic year, so at the end of each seven Sabbatic years there succeeds a Jubilee year.
....(snip)...
The legislation concerning the year of Jubilee is found in Leviticus, xxv, 8-54, and xxvii, 16-24. It contains three main enactments:
* rest of the soil;
* reversion of landed property to its original owner, who had been driven by poverty to sell it; and
* the freeing or manumission of those Israelites who, through poverty or otherwise, had become the slaves of their brethren.
Now the sabbatical year says
http://www.newadvent.org/cathen/13289a.htm
Sabbatical Year (Shenath shabbathon, "Year of rest"; Septuagint eniautos anapauseos; Vulgate annus requietionis). The seventh year, devoted to cessation of agriculture, and holding in the period of seven years a place analogous to that of the Sabbath in the week; also called "Year of Remission". Three prescriptions were to be observed during the year (Exodus 23:10-11; Leviticus 25:1-7; Deuteronomy 15:1-11, 31:10-13):
*The land was to lie fallow and all agricultural labor was to be suspended....The prescribed rest was for the land, not for man. Hence work other than agricultural was not forbidden, nor even work in the fields which had no direct connection with raising crops, such as building walls of enclosure, digging wells, etc.
*No crops being reaped during the sabbatical year, the payment of debts would have been a great hardship, if not an impossibility, for many. Hence the creditor was commanded "to withhold his hand" and not to exact a debt from an Israelite, though he might demand it of strangers, who were not bound to abstain from agricultural pursuits (Deuteronomy 15:1-3, Hebrew text). The Talmudists and many after them understand the law to mean the remission of the debt; but modern commentators generally hold that it merely suspended the obligation to pay and deferred the creditor from extracting the debt during the year.
*During the sabbatical year the Law was to be read on the Feast of Tabernacles to all Israel -- men, women, and children -- as well as to the strangers within the gates, that they might know and fear the Lord, and fulfill all the words of the Law (Deuteronomy 31:10-13).
*The law concerning the release of Hebrew slaves in the seventh year (Exodus 21:2 sqq.; Deuteronomy 15:12 sqq.) is wrongly connected by some writers with the sabbatical year. That there was no special connection between the two is sufficiently shown by the requirement of six years of servitude, the beginning of which was not affixed to any particular year, and by the law prescribing the liberation of Hebrew slaves in the year of jubilee, which immediately followed the seventh sabbatical year (Leviticus 25:39 sqq.).
So that must be where I got the seven year cycle from - you served seven years for debts you could not pay, starting from the time of sentencing, not from the beginning of a sabbatical year cycle.