I try to write 10 minutes a day. This seems like such a small amount of time but the idea is that by making it so small and inconsequential we do not put it off and sabotage ourselves. Everyone can find 10 minutes to sit down and write? You can't be that busy that you can't find 10 minutes in your busy day. But once you start writing you will often find an hour or more flies by from that initial tiny 10-minute target.
A great tip is to get an old fashioned hourglass for your desk. The sight of that sand flowing through will give you impetus and write, ...although make sure you don't stop when the sand of time is done because that will defeat the purpose. Some find the hourglass useful some don't.
Check out this article by fellow writer Janz for further reading.