There's no reason why there shouldn't be an option to choose from a provided list of possible times at the beginning of each game. There could also be a check box for "always use this time limit". Against NPCs there's absolutely no harm in this and if you still want to play with the current limit then so be it. In player versus player scenarios then a time limit would have to be agreed. If you can't agree then fair enough, don't play one another.

What many people have overlooked is that not everyone has the same mental or physical abilities to play in a meaningful way within a short time limit. By having a fixed, short limit they are being either excluded or put at a significant disadvantage. Surely if there is a way to make everyone feel included it should be implemented.