This is a great idea, my mom used to actually forget that I was on timeout. So I'd be sitting, and sitting, and sitting, etc.

As is I use the timer on my phone, but good to keep it consistent one way or the other.

My son is a little young (just turned 2) to be told to go and sit somewhere on his own and be expected to stay put. For time outs I have him sit on me and I explain calmly why he's getting the time out. At the end of the allotted time (2 or 3 minutes depending upon how long he takes to calm down and understand why he's in timeout) he says sorry. Rather than have your kid sit somewhere on their own and be ignored, I think it's more helpful for them to understand why they have to sit still.

1-2-3 Magic

http://www.parentmagic.com/

If you're consistent, it really is magic.

I just love how calm the child in these pictures is...

Look how sweet he is, thinking over what he's done wrong.