The most common question I am asked by people who are interested in changing their accent is, "How long does it take?" My most common answer is, "Depends." It depends on so many variables, but the most important variable is how much time you consistently dedicate to creating a new speaking habit. The new Goals feature on Google Calendar can eliminate your excuse that you couldn't find the time.
The key to creating a new habit is to do that thing, no matter what it is, consistently. A popular notion about the amount of time it takes to do this is 21 days. However, it's really not such a clear answer. In this 2009 study by Lally, Jaarsveld, Potts, and Wardle, the time can range from 18 to 254 days. Creating change in a habit to reach a level of automaticity (you do it without having to think about it) takes a longer duration of time, but what is key is consistency of performing that action. This is true for diet, exercise, and of course I'm going to say...speech.
It's much easier to set a goal than it is to keep a goal. From my perspective as a speech coach, I listen, observe, evaluate, and ultimately, identify goals for an individual that will help them reach their long-term goal of how they want to sound. From the individual's perspective, they now have information they didn't have before; they have an awareness of what they can work on and I have provided strategies for how to practice and improve. That's the easy part.
The most difficult part isn't just reaching the goal, it's keeping the goal on a consistent basis. Missing a day or a practice session isn't going to make a significant impact on reaching the goal, so if that happens, so what, don't miss the next opportunity to practice. The more complex the behavior is that you are trying to change, the longer time you should expect to practice. Speech is incredibly complex, so you should plan on how to make consistent practice a part of your daily routine for a long time. Of course, there are short and mid-range goals, but you should think long term for your ultimate goal.
I've been searching for the perfect reminder app, but I haven't found that golden ticket yet. There are so many reminder apps available for free and not free, that it really depends on your preference. However, since I run my life by Google Calendar, I'm happy about this new feature they added this month, "goals." This article in How To Geek has a good description with lots of screen shots. I already had Google Calendar on my Android phone, but needed to downloaded the (Google Calendar) app to get the goal feature.
Once you choose to add a goal to your calendar, it really doesn't matter which one you choose, you can always choose "custom" from any of the options and name it anything you want. The benefit of adding a goal through Google Calendar is that it will schedule and reschedule your practice sessions around your events on your calendar. It finds the time you have available, so you have no excuses that you "didn't have time."
For reaching speaking goals, try to work in at least 15 minutes a day. Even this short amount of time makes you focus on your speech for those 15 minutes, but really, it will increase your awareness long after that. If you start your day with 15 minutes of some vocabulary practice, you're going to be more aware of your pronunciation of those sounds and words when you say them later in the day. The practice doesn't have to be boring lists and routine. Do something speech-focused that will benefit your day:
Remember, setting the goal is easy. Reaching the goal is possible. Keeping the goal on a consistent basis is how you achieve it. The key is to take life day by day, don't worry about a missed day, and hit "reset" anytime you need to and just start again.