Severance offer, what should I ask for? - what worked for you?

I was offered 15 weeks of severance after 15 years as a hotel front-desk agent, and I have a few days to sign. I worked as a hotel front-desk agent before this. I do not want to leave money on the table. Did anyone negotiate and succeed?

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