You should be able to leave it for about 2-3 weeks without starting, if it's any less I would say its the battery on the way out (being 05 if it has the factory fitted battery it would be on its last legs now). But if the owner is telling the truth (rather than just admitting its needs a new battery) and it has been happening since new I would not think its acceptable, if its still under warranty make sure it gets fixed.

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Range Rover Evoque Si4 Dynamic