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:
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.