Dd started sleeping through at about 10 months. Then she had a couple of months at around 18mo, when she was awful at night again. And then she was fine till nearly 3yo (when ds was born) when the disturbed nights started again.
Ds started sleeping through in December, aged 22 months. Dd has decided to sleep through as well.
This last month has been such bliss that ... we've decided to ttc no. 3.
We have tried everything to kept them to sleep over the last 4.5 years, and I concluded that I should stop trying to fix their sleeping problem and work out how to survive until they grew out of it.
My main survival technique is to kick dh, mutter "your turn" and stick my head under the pillow.
My next baby is going to be the Incredible Sleeping Baby. Honest.
Good luck.