Are there any time limit per move in Ticket To Ride?
There is no time limit for each turn and etiquette for turn length depends on the group that you are playing in. The best suggestion that I can make for you is to remind your wife that she can start planning her next turn while others are playing. While things will change based on other players move not every action they take is going to impact her choice.

But one thing to remember that based on the game the actions of the previous player can drastically change what you are planning to do and games can last 1-2+ hours so this is not a quick game.