It seems to be ten turns until you start to see time decay, this however unfortunately seems to be the case from normal to marathon. So its particularily painful in marathon where everything takes much longer to build.
You can keep it alive if you switch back to it every ten turns for at least one turn. I've done this in a game where the tech rate was so slow that the only build was a wonder which I didn't want to start so early. So what I did was build towards it while I had nothing else to build and then switch back to it every ten turns when I was building other things.
I've also seemed to notice that decay kicks in on some other levels, either based on total hammers in the quene or total projects, or both. Meaning if you have two things in the quene you are switching between and then get an engineer and rush a third item you will see a loss on the other two items. It could be that with one other item its 10 turns till decay, but if you have two items it could be 5 turns before they decay. This is just a guess and it sounds broken so I hope this isn't the case.