Backing up data with Time Machine and iOmega

I haven't backed up my data with an iOmega external hard drive and Time Machine in three months. As soon as I plugged in iOmega, the screen went to the purple outer space background with a box indicating a list of options to be backed up. I clicked "All My Files" rather than the default "Documents" because I have been working extensively on iPhoto and iMovie. That was an hour ago. The screen has not changed. I can feel the hard drive whirring, so something's going on. How long should it take?