Netbeans will generate a signer certificate which life for six months (based on output warning it prompt: The signer certificate will expire within six months), anyway, there are ways to extend it either by signing your jar file using java keytools (tutorial available here), or change the configuration in jnlp.impl.xml file which generate automatically by Netbeans IDE when you build your project by “Enable Web Start” checked.
Here you go:
1. Look for the genkey node
2. Add validity value into genkey node, for eg: genkey validity=”365″ means 365 days.
3. Clean and rebuild your project in Netbeans.