Whether you should use a lifting belt depends on a variety of factors but generally speaking, you can get away with not using one – not even for deadlifts or squats – for a long time as long as you perform these exercises with the correct form. That said, you should never compromise on form, even when you wear a lifting belt. They can give you some support but won't be able to undo the damage you can do by not activating your core before heavy lifts.
As an example, and this is definitely not a rule of thumb, just a personal anecdote, I can do four sets of 12 deadlifts using 1.5 times my body weight without using a belt. The amount you can lift without a belt can vary from person to person, and the most important thing you can do is to listen to your body: once you feel a strain on your lower back, start wearing a belt. Especially if you do deadlifts/squats often (e.g. you're a powerlifter).