Nectar is a single-use Item. It can be applied once every 21 days. When you use it, all Units in the Infirmary gain an additional 14 days to be revived.
Nectar affects Units that are already in your Infirmary when the Item is applied. The units you lose afterward will remain in your Infirmary for a regular period of 7 days. They will affect the timer.
There is no need to worry if the timer shows 7 days or less. The Units that were in your Infirmary when Nectar was applied will be there for 14 days, regardless of what the timer shows.