I have been flying from Europe to Australia with Air China and the layout on the outbound is 13 hours 40 mins. Have no problem with through check-in of bags. Arrive 11:25am in the morning and leave 01:05am the next day.

If SQ can not through check the bag to the final destination, ask them to check to Singapore only (if you have more than 3 hours in Singapore otherwise don't bother). in Singapore there is Air China transfer check in desk where they would be able to pick the bag for you and recheck for you.

Also call Air China in advance to make inquire about transit hotel. If your ticket is issued by Air China, you should be able to get a free transit hotel. Otherwise you will have to stay in airside until your next flight.

Good luck.