You asked how common it is and no one else has given location information, so I will. 

I'm in the intermountain west area of the US, and around here it's extremely common to say "I'mma" or "I'monna". In speaking it's considered casual, but not so casual I wouldn't use it when talking to a coworker, and I probably wouldn't even notice if a coworker used it with me unless I was specifically looking for it. I would try to avoid it in more formal contexts, such as a presentation at work. However, in places where it's not common, its use would probably sound "rural" or "hickish".

In writing, it's extremely informal. I would never use it in an email, and in text only with my close friends and even then only when I'm being purposely informal.