It's recommended to leave them as long as they are under the weight requirements for rearward facing. You can get car seats that go to 12kg rearfacing, which usualy takes them until at least 12 months.

That said, I turned 'early', about 7-8 months. Would have liked to have left her rearfacing for longer but Jazz was a boofer, and her seat only allowed to 8kg rearfacing and at the time I didn't realise that this was quite a low weight and there were others that allowed a higher weight.

In Sweden it's recommended to 4 years, but they also have specially designed car seats that go up to 25kg rear facing.