check out 3GPP TS 04.60
UL TBF release can be done with 2 dfferent mechanisms :
1/ without extended mode
UL TBF is released T_DELAYED_FINAL_PUAN (about 400ms) after last block has been uploaded.
2/ with extended mode
it is released T_MAX_EXTENDED_UL (about 2s) after last block is uploaded. This mode is done if activated in the BSS + if the MS supports it (Rel’4 MS onwards)
DL TBF is released after a “delayed” time defined by timer T_Delayed_DL_TBF_Rel (equals to T_NETWORK_RESPONSE_TIME = 1.6s in alcatel)