For a classic green bean casserole, the usual bake time is about 30–40 minutes in a 350°F (175°C) oven, plus a short extra bake after adding the crispy onion topping.

Standard bake time

  • Most traditional recipes with canned or blanched green beans and cream-of-mushroom soup bake 30–35 minutes at 350°F , until hot and bubbly around the edges.
  • After that, the casserole is usually topped with fried onions and baked another 5–10 minutes until the topping is golden.

If your casserole is cold or made ahead

  • If you assembled it ahead and kept it in the fridge, add about 10 extra minutes so it heats through the center before browning the topping.
  • Check that it is bubbling around the sides and hot in the middle before serving; if not, give it 5–10 more minutes.

Variations that change the time

  • A very quick version that is pre-heated in the microwave can bake in the oven for about 10 minutes at 350°F, just to melt cheese and brown the onions.
  • From-scratch versions with fresh beans and homemade sauce may bake at a higher temperature (for example, 425°F for about 20–25 minutes after the stovetop step) but total oven time is still usually in the 20–35 minute range.

Simple rule of thumb

  • Start with 30–35 minutes at 350°F , uncovered, then 5–10 minutes more with the topping.
  • The casserole is done when it is visibly bubbling and the onion topping is crisp and lightly browned.

Information gathered from public forums or data available on the internet and portrayed here.