So long as they are kept sterile and well sealed, then they should be good for years. However most bottles don't seal really well and you will lose some water due to evaporation which will somewhat increase the concentration of sucrose or other ingredients. But if you are sampling from the same bottle repeatedly you are also likely to introduce contamination over time, so be cautious.  

Thank you very much for the reply!

You can increase the stability of a Sucrose solution by adding ~1 g salicylic acid.

as per my knowledge, sucrose solution will degrade with the passage of time.