Even if your plan is to leave the Company within the next 15 months, it still benefits you to sign, becuase it wil become a bargaining chip you use for the next employer. And you never know if you can find a good opportunity.
You don't need to sign immediately, they will give you time if you request. You probably should request, because it make you look better.
Theoretically, you could negotiate with them
1) if you plan to leave, you may negotiatte with them for getting raise instead of retention bonus. Most likely they won't accept, and that would tell them you intend to leave.
2) request for higher retention bonus if you intend to stay here. But that may leave a bad feeling to them.
Both depending on how important you are to them and how well you handle conflicts.
Hope this helps.