How far is too far to own rentals?

Just what the title says , what is too far to own rentals distance wise?

I'm sure that in some cases you might be able to own them out farther if they're in groups.

So , for those of you that OWN rentals (I can't say I want people that don't own any) how far is too far for you to own rentals?

Is there a distance if you have a PM?