Giving infinite timer has good and bad sides. Score could be awarded as an inverse of time spent solving the puzzle (a formula that gives diminishing returns on used time) and it would allow for a less strict limit for reaching the solution. On the other hand, the timer is presently the only fail state in the game. If the game cannot be lost, a higher score can always be achieved by playing long enough, which would make scoring a little redundant. But this could be solved by letting the player "win the game" by solving N amount of levels instead of allowing infinite gameplay. In other words, the game would have a theoretical maximum score that cannot be passed. It's something to think about.