If there is no reason to do ghusl for a Muslim, such as in the possible case there is semen discharge or sexual intercourse. How often should one do ghusl to keep clean? How long could go without ghusl if they don't discharge semen or have sexual intercourse for a long time?

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Ghusl as ghusl is required only if there is a cause, such as discharge of semen, or sexual intercourse…etc.
If there is no reason for ghusl, then there is no meaning for ghusl. However, you can have a shower any number of time a day, but you have to perform wodu (abalution) to be able to pray.