The manufacturer says to use within 10 hours.
Many people choose to ignore that advice because someone unqualified told them it was fine to do so. If people inject their ewes before lambing and give at least one dose there presumably is some cover and in small flock possibly it is enough.
Personally, I can't see why people ignore the advice but that is up to you.
The only post I have seen on here, which unfortunately I can't find now, by a person qualified and working in the industry explained that the solution the vaccine is held in deteriorates after 10 hours and the vaccine is no longer viable, that is why the advice is to use within 10 hours of opening.
Some people's vet says it is OK to use after the 10 hours and some people's say not. So, who is right? If I take my animal to the vet and purchase medication I would like to think that they give me something in date and give my animal the best possible chance.