If your employer does not have a written severance policy, this can be a hard question to answer. Typical severance benefits vary widely from industry to industry, and depend on the position. Many employers do not offer severance at all.
The best way to think about the amount of severance you are offered is by comparing that amount to what you are being asked to give up- basically, the release of legal claims. If you don't believe there was anything unlawful about your termination, or that there were any other legal issues during your employment, then even a modest severance is likely a fair exchange.
If, on the other hand, you have questions about your termination, or believe you might have valid legal claims against your employer, it is a good idea to review your situation with an employment lawyer before signing. Doing so does not mean you have decided to sue- rather, it is simply a step you can take to understand the value of what you are giving up in exchange for the severance amount, so that you can make an informed decision.