It is best to quit when you start to see signs that it is becoming an addiction. You can notice if gambling causes anxiety, stress, or discomfort. Generally, you tend to have excessive losses, so I recommend you escape from that situation and focus on other things to change your behavior. Generally, if you stop in time, you will avoid falling into addiction and be able to play again in a controlled manner.
The problem is that it's difficult to stop on time when you are making Profit, a lot of addicted gamblers I know got hooked because they made a lot of Profit when they started gambling. Addiction doesn't start in a day, it can take days, weeks and months. When it becomes a repetitive pattern it means that your brain have gotten used to it. When you feel anxiety when you are gambling because you are scared to lose then there's a problem
The truth is that people know it when they are gradually becoming addicted and like you said, addiction doesn't just start in a day, it takes a graduall process and by right the individual is supposed to know that they are getting addicted because they must have started doing some abnormal things like chasing losses, gambling with the amount they can not afford to lose and also not being able to stay out of gambling for one day. I once said that a gambler should always reflect on their pattern of gambling all the time, it will help them quickly realize it when they doing wrong thing.