I plan on about 6 hours of work in a regular day. That's project work, not answering the phone, email, making arrangements for workshops or consulting or speaking, or invoicing, or any of the other things I do. Nope, that's just project work.
The other half of that question is how many regular days do I have in a week? Either 3 or 4. I've had weeks with fewer “regular” days, where I could only accomplish 2 or 3 hours of work. This week, I've had 2 regular days (6 hours of work days) and 2 irregular days, where I accomplished maybe 3 hours of project work.
Since I make my own commitments, this usually works out. But not always. When it doesn't work–when I don't have enough project time in a week, I'll work on the weekend. I hate to do that because I don't have enough brain cells to work effectively the next week, making fewer regular days the following week.
If you know how much project time you have day-by-day, you have a much better chance of estimating your work on a project. So when you think about estimating your time for a given task, think about what you can accomplish in a regular day. And think about how many regular days you have in a week. If you only have one day a week where you can accomplish 6 hours of work, and the rest you can accomplish 4 hours, plan for the 4 hour every day. You won't have less to do over time–you'll have more.