Optus has generally shut down their cable network within three months of the NBN being declared active in an area. Might be a little longer but basically don't put it off and get your NBN transition sorted now.
If you are cautious then you can get another RSP to install the NBN for you. This will mean your current Optus cable remains in place while the NBN gets installed (The NBN uses a different physical cable to the Optus one). Just select an RSP with a month to month plan. After you're happy you can switch back to Optus or stay or whatever.