Hi Nguyen,
it is is easy to set up auto approvals with the standard delivered functionality, but it is not possible to set up an auto reject function with the standard configuration and functionality available.
I do feel it is achievable via some deep customisation, which could involve making and invoking some custom ABAP class or function module. I dont know if BRFplus will achieve this functionality. You may need the support of a good ABAP programmer.
Look forward to anyone else's thoughts or even a solution!