How much time does it take to walk 12km?

Under the conditions you described I think that it shouldn't take more than 4 hours, if you take it easy.

When you walk at a normal easy pace you do approx.3 to 4 km/hour.So it will take you 3 to 4 hours.

If you walk 1,5m/sec than you need 8000 sec which is ~133 minutes + add how much breaks will you take.

It depends a lot on the terrain (uphill vs. downhill) and whether you are walking fairly fast or taking your time.  Generally, when hiking, a long full day of hiking is something like 8 miles... I think that you could do 12 km in something like 5 or 6 hrs.  It's difficult to say though.  It also depends how many breaks you take and how long your breaks are.  I would give yourself the day though, and you can take your time and you'll be fine.