As long as you want. If you are the one holding out, then when ever you are ready. If you are the one being held out then as long as you are willing...meaning when you are no longer willing the relationship ends.
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Every relationship is different. It depends on the two people involved and how long you have known each other. I would wait until you get to know each other for a few months before I ask him to be your boyfriend. Relationships are more successful if you are friends first and don't rush into anything romantic until you know each other.
get to know the person better
ask him/her about their hobbies and other stuff
then when you know that he/she like you
ask him/her out