I've written several detailed explanations of this, but I can't find any of them in a hurry, so here goes.
It's a common question. Sous vide comes a lot down to personal preference. With a steak there are a lot of variances to take into account, the most obvious of which is thickness. How thick is a steak?
Sous vide, like all other cooking methods, is about heat absorption. With sous vide though, because you are cooking at the temperature that you want the finished product, the absorption of heat is slower. For an inch thick steak, the time to get it up to the temperature of the water will take around an hour. For a steak that's about an inch and a half thick, that'll take closer to 2 hours. One that is 2 inches thick will take nearer 3 hours to reach the same temperature at its core as the water.
Sous vide is a forgiving cooking method. There will be little noticeable difference in between a steak cooked for 2 hours and one cooked for 4. So, you will see a lot of instructions that list a range of times for cooking. Steak is commonly listed as 2-4 hours.