Your market may vary, but...
Before you advertise you should already have a good idea of the market, and you can watch ads for a while to see how many weeks they are repeated to get an idea of how long you can expect.
If you price appropriately, you will get response. Now you have to learn how to "read" that response. If you get no calls, no views, you are likely too high or there are other issues. If you are swamped with calls, you priced too low. I would say over the long term, on average, you will talk to 10-12 people in order to actually show to 4 or 5 prospects, and get 1-2 good quality applications; 7-10 days would be an average vacancy period, assuming minimal change of occupancy maintenance.
It really is dependent on your local market, and the specific unit, as I have had MANY same day COO, and MANY six weeks or more, not counting actual repair time. Your writing style for ads can make a difference; your phone and/or in person presentation will have an effect. The time of year and weather matters. Even what part of the month you are in will affect the response.
Typically, if a given unit is "average" and I don't get people inside to view within 2 weeks, I'll drop price a bit. If, OTOH, I get lots of tire kickers, but no bites, I'll hold the price and address any issues that have come to light as objections.
I am of the opinion that a given property has a certain value, and I will not undercut that value just to get a tenant. I realize that investors often feel (and, correctly) that a day of lost rent can never be earned again. I have observed however, that the quality of the tenant is more important than a few days rent, and much more valuable in the long run. I'm willing to wait.
Another factor, I do not show properties while occupied (unless the tenant is breaking their lease, and has authorized me to show), in fact, I rarely will show a unit that is NOT in move-in condition. Most people can not "see through the dirt", so you lose potential good prospects, and waste a lot of time. Besides, it is just not a good idea from a liability standpoint to be in an occupied unit with strangers.
I DO start advertising before it is vacated, and schedule showings for the day after it is scheduled to be vacant. If you have digital pics, you can easily package a simple PowerPoint presentation to email prospects. You can also save yourself a lot of wasted time by "pre-qualifying" EVERY prospect on the phone. This has the added advantage of providing supporting documentation of your administration of the Fair Housing laws.