12 months or 20,000 miles whichever comes first.
However, I would never do 20,000 miles between oil and filter changes. An annual service is what I'd recommend, and no more than 10,000 miles before changing the engine oil and oil filter.
See
http://carleasingmadesimple.com/business-car-leasing/vauxhall/astra/service-interval-miles/
ChicksFan
June 2013