Vacancies can completely wipe out any cash flow on your investment property. A good property manager will act to keep down time as low as possible. A good property management companies will keep a list of potential tenants that they can show existing properties to. Make sure it's written into the lease that you can show the property to future tenants while the current tenant is still occupying the property.
The most common delay we run into is when the past tenant has left the place trashed. In this case the work needs to be performed as quickly as possible. Performing an inspection a month or two prior to the previous tenant moving out can help you know how much work is going to be needed to get the place into rent-able condition. In most cases we order the work for the week of the move out and then set up showings as soon as the work is far enough along to be presentable to future tenants.
For properties left in good condition, we've been able to minimize vacancies to less than a week. With all of this the biggest factor is timing. It's important to list, show, schedule repair, show the property, and get leases signed at the correct time.