I won’t be able to give you the spec now, but yes, there is a timer to switch from listening the PACCH (PDCH) to the AGCH & PCH (CCCH).
But the mechanism is rather complicateddepending on the situation . Are you sure that the DL TBF is fully released when the MS is sending UL data ?
Once the MS has finished sending data in UL (and there is no DL TBF neither active nor delayed), the MS will enter a specific state of “extended phase”, if a certain timer is > 0. (I think the timer is called T DELAYED TBF POL… or T RESPONSE NETWORK TIME… but i can never remember this timer… I’m not sure)
On the last useful UL RLC blocks, the PCU doesn’t acknowledge the reception (in PUAN, FBI = 0 instead of 1), in order for the MS to remain “aware”.
During this time, the MS remains on the UL PDCH, sending “dummy” blocks in UL, if the PCU sends the USF.
I’m sorry, it’s not a very precise description, but i need to check the documents to be more helpful. Tomorrow morning, i’ll do that 🙂