Can anybody suggest me how many days a keyword comes on first page of Google ?

This changes instantly.Some will remain on the initial page for years while others show with regard to for only minutes and drop just like a stone.

It will depend on the level of competition.If key word competition can be low then it will take approx 45-60 a short time or if your competition can be so large then it’ll take a lot more than 120 days on frequent working.So you must use extended tail key terms for optimizing these high opposition words.