Not sure which version you are using, but support for dynamic resolution of timer properties was added on November 26th, so it should be included in the latest CR1 release:
https://github.com/krisv/jbpm/blob/5.0-CR1/jbpm-flow/src/main/java/org/jbpm/workflow/instance/node/StateBasedNodeInstance.java
(check the resolveValue(..) in the createTimerInstance method)
If you are using the latest version, but still have issues, could you give an example process + java file that show this issue?
Kris