It's easy enough to include two modes for a match-3, untimed and timed (i.e. complete the level before the timer expires).
But I'm not entirely convinced that it's a good idea to include both modes. The problem is that it becomes hard to balance them so that they're both challenging. Usually, the timed mode is challenging, and the untimed is not - you can play the latter for an extremely long time, only losing when you get an unlucky board situation.
Bejewelled has both modes, JewelQuest is timed only.
I'm inclined to go timed only, just so I can better balance things for that scenario. But of course, this could alienate some users who only want to play at their own pace.