There is no specific time to give your loved one an eternity ring. Some people give them to celebrate a particular anniversary, others wait until the birth of their first child. Down to the individual, i'd say.
i've been hinting ever since i had my first baby in 2010. As she didn't survive it didn't really seem appropriate to celebrate her birth with a ring. Just before xmas, this year he took me into a jewellers to look, but the thing is, although i see a nice ring, i also see a mortgage payment, or a holiday