It's not your 'fortune' that's reduced by this amount, but your income. If you fill a tax return (foreigners earning >120,000 or home owners or C-permits) you can deduct the amount you pay into 3rd pillar from your taxable salary before you calculate your tax. For those not in such a position, I'm not sure if you can 'claim back' against tax.

Deadline will depend a little on whose 3rd pillar product you buy, but typically sometime around mid-Dec, to allow them to process etc.

Also for those paying additional into BVG (2nd pillar), the November deadline sometimes mentioned is also flexible according to your pension co. (mine says Dec 20th).