Hi Julia
The TRA doesnt actually take that long to come through. Compiling everything for the TRA is what takes time. Ours took about 2 months I think.
When you receive your visa you have 12 months to enter Australia from the date of the Police/Medical checks so you really need to keep an eye on that. No point in rushing things.
The TRA doesnt have an expiry date - heres a bit more information here (you'll need to copy and paste it):
http://www.workplace.gov.au/workplace/Pages/ContentPage.aspx?NRMODE=Published&NRORIGINALURL=/workplace/Category/SchemesInitiatives/TRA/1SkillsassessmentforpeopleintendingtomigratetoAust ralia.htm&NRNODEGUID={98D6834A-A34B-4E3A-A763-F644E0924582}&NRCACHEHINT=Guest#10
If you've got anymore questions feel free to ask - we emigrated from the UK to Australia 14 months ago so I might be able to help you with further queries
Good luck
Rach xxxx