Over time erosion of the string with the bridge makes the last one to wear away. It is therefore advisable to make bridges out of horn or ebony, since these ones are the most resistant materials. On the contrary, bridges made out of camel bone last less.
On average, the life of a bridge is around 3 to 4 months if you play regularly.
When the bridge "erodes" too much, sound opens and looses definition compared to the initial sound. Pullings are less lasting. Then you need to sand the whole basis of the bridge for the sound to shine again.
This work in India is reserved to luthiers and there are very well up on this field. The relationship of musician and luthier is very close: as the jawari has to be checked periodically, the luthier gets to know each musician’s preferences.
For instance, Ravi Shankar was taking with him on his tours one of his regular luthiers to always keep the bridge ready.
This is a fact, but it is also true that we are talking about professionals who dominate the instrument in a field where perfection of sound in each and every stretch and vibration is extreme.
You can sand the bridge yourself and obtain satisfactory results (it is just a matter of patience), it is not difficult but some basic knowledge is required.
No one can ever do it for you without the entire instrument, since to perform the jawari, the Sitar is needed to keep on checking the sound that is obtained during the process.
Sitarsencat recommends you to learn to sand the bridge. There is an educational book, easy to understand, that explains step by step how to perform jawari (The Sitar, from Manfred Junius). We recommend you to get hold of it, and over time and with some experience you will learn to get the sound which best suits you and enjoy full performance of your sitar. Patience and good luck!