Motor manufacturers, having been moving towards timing chains because of reliabilty and longer service life - and reduced maintenance costs - are now moving back to belts.
There is no recommended replacement period/mileage for the timing chain in an Aygo and in a well maintained vehicle a chain will most likely last well beyond 100,000 miles.
In the latest engines the belt is making a return to favour because they now tend to be lubricated during operation, increasing reliability and extending life - up to the 100,000 or so miles now becoming more common.
You should bearin mind that 'maintenance free' doesn't mean the belt doesn't need replacing, but that it doesn't need adjusting etc. once fitted.