Originally Posted by DamianJ

DO NOT partition a drive you want to use for back ups. It is ASKING for trouble.

Hard drives are cheap as you like nowadays. I just got a 750 gig SATA for 80 USD.

I lost one year's worth of data once with a HDD fail, so now I have one ext with time machine back ups, and another one with a bootable SuperDuper mirror of my whole drive.

But to answer your question, time machine makes hourly back ups and then when your hard drive is full switches to weekly backups. So you just leave it alone basically and it manages your shit for you.

But don't partition a drive you rely on for back ups.