I use MS Project to plan most of my jobs (design/implement/maintain/document a database application with a web front end - about 800 objects in the db and 1200 files in the website) and have a set of 'rules-of-thumb' to estimate time to make changes:

1. Estimate how long each task will take based on past experience. Double it. If uncertain, double it again.

2. Change working time from 8 hours to 6 (my manager keeps reminidng me not to assume 8 hours!)

3. Add in known meetings, etc.

4. Look at the result and see if it look reasonable.

5. Add in the minor tasks you forgot about the first time.

6. Adjust as needed.

Even with the above, it's still sometime hard to hit deadlines! 🙂

I find that if I get 6 hours work done in a supposed 8 hour day, I've done well, typical is probably about 4 hours of real work.