The address remains reserved for 6 months (in case you rejoin) but one of the main reasons all Telcos offer an email service is because it makes leaving a pain. IMO You should migrate to a neutral online service (gmail?). They will have better security (2 step authentication), spam prevention and of course you can use them with any ISP.
Set one up now and have gmail grab any emails to your optus address into gmail. Tell people your new address. (Search back 12 months) and change any contact email for banks, websites, etc. that have contacted you in that period. Set up an auto reply in Optus that you have a new email address. After a month or two you should be largely transitioned.
Hey Rowdy0800 - if you're disconnecting your Optus service, we are sad to hear this. We can reserve your email for a maximum of 90 days once your service has been disconnected. After that, if you do have a Mobile Broadband service we can link your username to that. Hope this helps.