None. As a former Belgian diplomat, Jean De Ruyt is not subject to the EU revolving door rules.
Covington & Burling LLP is an international law firm with a major office in Brussels. As well as their legal practice, according to their website, they also "ensure that industry’s voice is heard in the EU legislative process and in administrative decision making. Our capabilities in this field are greatly enhanced by our senior European Policy Advisor, Wim van Velzen, who served with distinction for over a decade in the European Parliament, where he was Vice President of the European People’s Party, the largest political grouping in Parliament." http://www.cov.com/brussels/
“Ambassador De Ruyt, a non-lawyer, will work with Covington’s transatlantic government affairs team, which includes experienced lawyers as well as former senior policymakers. The team advises clients on a range of European public affairs issues, including the EU policy-making processes, functioning of the European institutions, development of EU legislation and accession of new EU members. Ambassador De Ruyt has particular expertise in the workings of the EU Council and EU institutions more broadly, transatlantic relations and United Nations development policy.” http://www.cov.com/jderuyt/
Before joining Covington & Burling, Mr De Ruyt also spent four months until January 2012 working as Special Envoy to Catherine Ashton, the EU's high representative for Foreign Affairs.
"Ambassador Jean De Ruyt is another influential Brussels diplomat who has recently been recruited by the lobby industry. His on-line profile at Covington & Burling indicates that he was closely involved in Europe’s response to the financial crisis and the resulting legislation at the European level and that he has also facilitated the resolution of a variety of state aid and competition policy disputes for Belgian companies. CEO considers that his move through the revolving door could provoke the risk of conflicts of interests. Covington & Burling are not registered on the EU's lobby transparency register so it is not possible to know who their lobby clients are."