There are different opinions when it comes to how often you should rosin your instrument, but the general rule is that you do it for every three to five hours of play. However, it still depends on how intense you usually play the instrument, how much you play it in a day, and what type of rosin you use.
Of course, the more you play it, the more you need to rosin it. Some types of rosin will wear off faster than other ones, so you need to pay attention to that. It takes a bit of experience to feel when the rosin is running low on your bow. Nevertheless, you would usually be able to feel the hair grip a lot less on the strings, to the point of sliding or making no sound.
Don’t forget to rosin your violin every time you’re playing it. The rosin you used yesterday may have worn off.