While watching the only trailer Super Mario 3D World has, the timer maximum is 477 seconds. Because the Mario has already started that stage I am going to assume every stage will have 500 seconds (no science rocket there). That will be exactly 8.3333.... minutes. That should me long enough as to run almost everywhere you want.
I also asked myself, Super Mario 3D Galaxy 2 didn't have a timer. How much did I spend on it during every stage? well, I don't have the game with me (i am in college), so I decided to check how much did someone else spent on several stages by looking at some random walkthrough on youtube. I was impressed the guy spent around 8 to 9 minutes playing 10 stages, that includes getting comet coins and the usual star at the end of the stages.
To my conclusion, there is nothing to be worry about. Yes, I can agree to a certain point that the timer can restrict some of our choices, but it also adds some difficulty by doing so. Replayability also. I remember Galaxy 2 being somewhat light on the difficulty. So there you go, what are your thoughs on it?