How often should clutch fluid be replaced?

[artificialist]

No, because that doesn't get rid of the junk in the hoses and slave cylinder. That old fluid can be even more foul than what is in the reservoir.

 

[mechtech2]

You can open the bleeder and just let it drain. use a hose and don't the fluid get on any painted surface. This will get fresh fluid quickly in the system.
Keep your reservoir full - don't let her run dry when doing this!
And DON'T touch the clutch pedal with this procedure.