Lemon juice is a natural bleach and that is why it is able to lighten hair. It is worth noting that the effects of lemon juice on hair will only be noticed if one exposes his or her hair to the sun after application.
One or two lemons are usually enough to lighten hair while ensuring that it looks natural. After application and exposing hair to the sun for about 20 minutes, the effect should be noticeable. The more times application is done the lighter the hair will become. However, it is always a good idea to wash and condition hair afterwards as lemon juice may cause hair to dry after some time.