The time investment required almost certainly varies by the system being implemented. As an example, implementing the Pomodoro Technique can take very little time - get a timer, and start working in timed intervals with short breaks. Inbox Zero can be started with relatively little time, if you start with the Inbox DMZ and go from there. A fully fledged GTD system can take anywhere from several hours to several days to get enough implemented to see results (or so David Allen states in his book.)
Implementing a productivity system will require time based on the complexity of the system, and the distance from your current state to the working state of the given system. Without specifics of which system you're considering, or how much organizational debt (the amount of disorganization you have to overcome) you've accumulated, it's hard to estimate any particularly useful number. I implemented Inbox Zero in an hour or two one evening, and saw results from it the next day. My co-worker couldn't pull that off - his inbox has orders of magnitude more email in it than mine did when I made the decision to give it a shot.
If you're trying to get results quickly, there are steps you can take to get some results sooner (Inbox DMZ is a perfect example) and move you towards improving productivity without a big investment before any return.
