The first answer to this for most players is when the strings in the racket break.
However, most thoughts are that you would restring your tennis racket as many times in a year as you would regularly play during a week, so 3 times a year if you play 3 times a week.
Other options are to string the racket at least twice per year, once for the spring/summer season and then again for the autumn/winter period.
Restringing a racket regularly gives the feeling of holding a new frame, it adds vitality to the racket. String tension can drop off rapidly in rackets and leaving them to break before a restring is done can undermine your playing ability without you even noticing it.
Advanced players regularly string their rackets every tournament or every few weeks to keep the feel-factor the same every time they play.
So, my recommendations are:
At least twice a year (Seasonal weather changes affecting the strings)
Restring as many times in a year as the amount of times you play in a week.
Restring every month if you are a tournament player.
Do not wait for a breakage before you get a restring!