My understanding is that you are obliged to file a tax return if you have over 120CHF income.

I came to CH part way through the year. Is this 120CHF limit pro-rated for time here?

Also, will the tax authorities tell you whether you are over the 120CHF limit or not - or is the onus on you to add up your wages for the year, pro-rate (if necessary) and then decide if you need to file a return?

You'll get a notice if they want to you fill out a form. May take years, but they let you know.

Do the sums. If you can save taxes by filing, then you could volunteer. Otherwise wait for them to ask for your filing.

It's not pro-rated, you need to earn more than 120K gross income in a calendar year for the ordinary taxation (with some exceptions in some cantons, like GE for instance, where it's 500K). Once you are above 120K, you will be subject to the ordinary taxation and this also for the following years, even if you fall below 120K! It's not a choice, it's a duty. Therefore you can wait for the authority to come, but basically if you don't hear anything, you should request the forms by yourself...

This is a bit different as I heard from deloitte,
I came here June 2009 and changed to local contract 1-OCT-2009.

But they told me I need to do tax filing even I got only 50K+ for 3months.
I think tax filing is pro-rata based ( 120K + per year.)