You don't need to keep taking the iron tabs while fasting. Your body will conserve what iron you have while fasting very efficiently. Fasting rebalances the system and you will absorb the iron from the raw foods you eat when breaking the fast and quickly regain healthy levels.
If you are worried about staying off the tabs for a few days it would be best to get a professional opinion from an experienced fasting doctor though. You can find one on the Advice or Fasting Retreats page here:
As for how long, that depends completely on you. Normally it would be best to fast to completion, that is the return of hunger after all fat stores have been used up. However if you really do need to restock some reserves you may wish to start with a shorter fast, say 7-10 days, then refeed on raw food for the same amount of days to give your body a better supply of nutrients to draw upon.