That depends on the Slingplayer client being used.
Slingplayer Mobile employs a maximum threshold for streaming, so its relatively easy to determine how many hours will burn your bandwidth. This post (https://community.sling.com/message/9071#9071) should reflect the information.
If you're using Slingplayer Desktop or Web Slingplayer then that will depend the average bandwidth stream. You can use the statistic information from the bottom-right of the client to determine the average consumption. Then use simple math to determin how much is being used. I put the equation is the aforementioned link for your reference.