I am a getting a similar problem, which in my case is not an estimate. Time Machine is really super SLOW. Time Machine told me that the backup needed to be rebulit. It is around 600GB. So I decided to start the new backup yesterday night at 11PM, assuming that overnight should be able to finish. After 12 Hours it only backed up 9GB!! It says that it will need another 22 DAYS !!!
I have OX Mountain Lion 10.8.2. I am backing up over WiFi g/n (in the same way I have been doing for the last 2 years). I have read in a cNET forum that after 10.7.5 version of MAC OSX Time Machine had problems with Spotlight, and that I needed to change something in Spotlight. Please advice what I need to do to signigicantly improve the performance of Time Machine.