You can fast whenever you desire, but best way to start is to prepare for it. I would eat good healthy Diet first and then start the fast with water only. Take one day at a time, for everyone the fasting period is different.
My first fast lasted three days, and then voluntarily I broke my fast. Second time, I was forced to eat in a day and a half. Third time it lasted for 22 days, which also I gave in to family members bugging me and the reality we all face (Well, some of us)to get back to work and get the paycheck.
Candida will clear with fast as well, but I do not know anything about candida so I would rather post on Candida Forum .