It makes sense to say, "My working hours are from 9 am to 6 pm."
But is it right to say, "My working hours is from 9 am to 6 pm."
My argument is that from 9 am to 6 pm can be treated as a singular subject, as is "butter and bread".
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Since you say 'my working hours', you are referring to each hour in which you work. The fact that they happen to come one after another and form a single stretch of time doesn't necessarily matter grammatically.
You could, for example, say 'my working hours are 9-10 o'clock on Monday, 11-12 o'clock on Tuesday, 9-10 o'clock on Wednesday...' etc and saying 'my working hours' would still be correct because there are multiple hours there, despite they're coming only one at a time.
Were you to use 'is', you would have to say something like 'my shift is from 9-6pm', as the word 'shift' bundles all those working hours into a singular entity, whereas 'working hours' remains plural.