I guess you'd be heading to Rovaniemi? 3 days would be your max as there's not much there once you've done the Santa stuff and sleigh ride. It's not a big town at all.
And prepare yourself for Finnish beer prices...
It's a lovely place though, well worth doing once in your life.
I'm down in Helsinki, but give me a shout if you need anything answered about Finland, been here a while now.