See how it goes, if you feel without energy you can integrate the fast with minerals, see also the posts of MOG in Ask Barefoot herbalist, how he feels while on fast is how you should feel too, very energized. He also takes some super-food to provide his body with minerals. If you feel exhausted please don't continue, the body wouldn't have enough energy to cleanse and it would be all useless effort.
Something you can consider in that case is what I'm doing myself (without limit of time) that is eating only for dinner and leaving the body fast for 18 hours, during which I drink all urine I make and a birch leaves tea (for the minerals). It is a bit difficult the first three days, after which it is a piece of cake.