It depends on your string tension.
Every player will have their own preferred string tension due to different play style and or technical skills. You can count how many times you played until you feel your string tension is not on your preferred range anymore. Then you just change the string regularly using that schedule.
For example, my preferred string tension is 25 lbs. I always strung my racket slightly higher than my preferred tension, because the tension will decrease overtime. It allows the racket to be in my preferred tension a lot longer at around 24 lbs. However, one can say that 25–26 is fairly high tension. The string will break before the tension degraded. So what happens to me is, I never keep track exactly when I should change string. It will snap by itself, that means I have to change the strings. On average, it will lasts 35 to 40 sessions depending on how much you play. For me, this is about 3 months, if I play three times a week, 3 hours per session. The benefit of using high tension is, when your string tension goes bad, it will snap off on its own.
Now, if your string is not on high tension, let's say around 21–23, your string unlikely snap on it's own. For example, I have a racket strung at 23, if it didn't break at 22, then it will gradually decreased its tension. It can last a year or more without snap. However the tension goes off. It means it might not bounce properly when defending, or loses its elasticity. For this kind of setup, you should mark by yourself when does your string becomes bad ( example 4 months )
, because it will not snap by its own. Since everyone have their own preferred tension ranges, only you knows which tension is appropriate for you.