If you need to tackle a challenging, yet unfamiliar goal, I recommend starting at 20% and moving upward. Figure out what you feel you could reasonably accomplish, cut it down severely and then slowly increase it as you get used to the habit. Some examples: Writing. Think you can do 1000 words a day? Make it 200 the first week, 250 the next, etc. Exercise. Think you can do an hour a day at the gym? Start with 15 minutes. Reading. Want to read a book a week? Try reading 5 pages a day. Language learning. Start with a twenty minute lesson, once a week.
Start slow and build up as you get more comfortable.