It looks like you have the expression to call your length variable written incorrectly. It's written in your length field as 'vypnuto' when it should be showing Elektropole.Value('vypnuto'). Also, I don't believe you have to stop the timer in your On Timer trigger if you've set your timer to No Repeat.
There used to be a bug in the Timer behavior where you couldn't start the Timer again from an On Timer event, but it was marked as fixed. It could be that it's not 100% fixed.
At any rate, it's easy to set up your own timer, so you may want to consider doing that. I have an example here if you'd like to take a look:
http://dl.dropbox.com/u/529356/customtimers.cap
v0.99.83
I've commented it so you can see how it works. It uses TimeDelta though, so if you're not familiar with the expression then I'd recommend checking out the wiki article.