Question about my PM marketing my property

[Christopher Davis]

I'm kind of a new landlord. I have a tenant moving out in about a month. (3bdrm house, Nashville.) I asked my property manager when she could start marketing the property for a new tenant. My assumption was that she could start showing it asap (while it is still occupied), to reduce the potential vacancy time. She told me she would start showing it when it is vacant and clean and ready. So then who knows when it will rent. It would be at least a month and likely two-three months. 

Question is, is this standard? To wait until it is vacant to even start showing it or advertising it? For three months I'll be out 6k in mortgage. I don't know what is standard and if I just eat my lumps and that's how it is. Or am I being pushed around? Should I firmly tell her what to do or just let her decide and go with the flow?

Thanks for any guidance.