Can I use "onwards" instead of "on end"?
No, it would not make sense to say "onward" here instead of "on end". "On end" is an idiom that means "continuously". So the expression "for days on end" means that something happens continuously for multiple days. This expresses a different meaning than if you were to say "onward". Ex: I haven't eaten for days on end. Ex: Sometimes she studies for days on end.
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