There are 3 levels of difficulty. The easy ones are good to start with to understand how the hints are written. I've done a dozen of the 'normal' puzzles and they took between 9 minutes and 45 minutes. The 45 minute game may have been shorter because - after making a mistake, a time penalty is added.
I think puzzles are generated randomly. Also, at the end of one game, I noticed that there were 2 possibilities for the final 2 cells. But there should be a unique solution for each puzzle. That's okay.
https://itunes.apple.com/us/app/six-logic-use-your-brain!/id344058289?l=es&ls=1&mt=8 In Spanish
https://itunes.apple.com/us/app/six-logic-use-your-brain!/id344058289?mt=8 In English
Six Logic: Use Your Brain! was originally published in December 2009. It's been updated a bunch of times, most recently in September 2014.