How many hours will viral particles remain viable once thawed?

If kept on ice, the viral particles will remain viable (with no appreciable loss in titer) for no more than 8 hours. Generally you see a decrease in titers if you refreeze a tube of virus that has been thawed. But if you keep it on ice before the virus is placed back in the -80C freezer, you will only see a slight decrease in the titer (approximately 2-fold).