It will depend on many things including how motivated to sell are you.
As a seller, how cooperative will you be in allowing the home to be shown? Will you allow a lock box on the home? Will you get the home in 'selling shape' (do repairs prior to putting the home on the market, getting rid of clutter, white glove cleaning, staging, etc.)? And probably most important when it comes to the amount of time you will need to sell: are you going to price to sell or sit?
The agent you hire will be able to show you the current average marketing time and selling prices for property in your area and let you know approximately what time frame they think they will need to sell at the price you choose.
Because agents do not control the market, and depending on the available pool of buyers for your home and if you feel the agent is working hard for you; if you home doesn't sell in the time frame of the listing, you can extend the contract date.
In the current market in my area, if I take a short sale listing I will not do it for less than 12 months and for a traditional sale, I like to get 6 months (as long as the price my seller picks is supported by the comps) and will do 3-4 months if I know my seller is priced aggressively.
Best of luck in your sale,