What is best time to negotiate salary with employer?

1. When you have moved up to a higher level position
2. When the company is doing well financially
3. When you get a higher salary offers

when the company is doing well financially and you are performing well above set targets.

If you are performing very well in written test and interview as well as employer is convinced wih your performance than you can negotiate salary with employer.

when we get a hike in other company

i totally agree with mr. amer

it is the better time to negotiate salery with company 

when you decide to leave the company

First you MUST know your reall value in the market within same field and same company in order when you cant have it in your company you can look in others which feels that you are valuable to it , but you MUST be in your Job as always and make your work but even more try to add value even if you look in other company as long you get paid by the end of every month till you find the best for you.