When speaking of traditional ways of selling (through a realtor), it's very difficult to predict how long it will take to sell any given piece of land. There are too many factors to consider - market conditions, how desirable the location is, the size of the land, how affordable it is, what the access is like - the list really could go on and on.
To get a quick visual check of the health of the land market in your area you can view the for sale and sold land listings on a website like Zillow. Filter 'Home Type' so you're only viewing Lots/Land. The red dots are currently for sale. Compare to the sold listings which appear as yellow dots in Zillow.
Roughly the same number of red and yellow dots in your area means there is a healthy demand for land and that when land becomes available, there is usually a buyer for it. When there are many more for sale listings than sold ones (more red dots than yellow), it's a sign that there aren't enough buyers for all the land that people want to sell in that area and it could take a very long time to sell land there.
To analyze further, you can click on a sold listing and check how long it was listed before it sold. Many, but not all, listings will have this information. To find it on Zillow, scroll down to 'Price History.' There should be a date listed when the property was listed for sale, and also when it sold. Some listings may have several dates where the property was listed, removed, and relisted before finally selling.
By checking how long several similar properties to yours were listed before they sold, you can get some idea of how long it will take your property to sell if it's priced appropriately and marketed well.