How much time is required to ply a good strumming?

It totally depend of your effort.. For me it took 2 months for strumming practice. The best way to learn the strumming is to play the chords/riffs without singing.. First try to play it while the music plays in the stereo or something. So that you can fully concentrate on the strumming part and you'll get an idea about how to sing while doing the strumming.. Through this way you can learn the strumming and sing part easily.. After learning one or two song, you'll automatically get an idea to strum for each song according to the tune. Hope this information helps you out... Best of luck.. :)