Again, this is entirely up to you. If you write everyday for creativity, it is best to write at least fifteen to thirty minutes, and writing for an hour or two can be very rewarding. If you are writing for Problem Solving, write until you have gained some perspective, or at least committed the words to paper so the next morning you can review and gain better perspective. If you are interrupted and you have not satisfied with your entry, go back and write some more. Once writing in your journal becomes a daily or weekly habit, you will find a comfortable length of time to write.
Many people like to write in the morning when they first awaken.
For someone who keeps a dream journal or a creative journal, this is a wonderful idea as it starts out your day by awakening your mind. If you keep a chronological journal, it might be best to write in the evenings when the day is still fresh in your mind. Some, especially those who write on a daily basis, will find themselves consulting their journal multiple times a day, even if it is only for a few minutes to jot down a few thoughts to ponder the next morning when they do their creative writing. When you write in your journal should become a habit with how often you write in your journal. Once you find a comfortable time and interval, you will find that your writing during those times will be easier.