If you are optimizing short keywords then it may take years depending on the competition, I would say competition is increasing day by day in Google mainly.

I would say optimizing long tail keywords would help in short period of time...
What is your opinion about it?

I agree with you dear. Short keywords are not helpful for SEO. Its take a long and long time to give results. Long tail keywords are very helpful for SEO. They give results in short period of time....

True said!
Many websites got adversely affected by excessive usage of long tail but Google has re-emphasized the usage of long tail post the panda update.
Long tail keywords include short keywords too and hence they can be of great help to help you rank high.
I would suggest you to use Long tail keywords smartly.