Severance offer, what should I ask for? - 5 months and counting

I was offered 18 weeks of severance after 18 years as a janitorial supervisor, and I have a few days to sign. There are 18 people counting on me, so I am trying to be careful. I do not want to leave money on the table. What did you ask for?

I asked for two more weeks and a neutral reference. They agreed to the reference and added a week.