SACCH for the SDCCH is used in exactly the same way as for TCH. It is used to carry system information msg/pwr ctrl command in the DL, and measurement reports in the UL.
Yes, the SDCCH usually lasts longer than 0.5s. In average, it lasts 3.5s. It lasts longer for SMS, and lasts shorter for LU or IMSI detach.