A lot.
With the clutch engaged, roll the bike forward and check the slack in the upper chain run. You'll want 1/2 to 3/4" of slack. More than that and you're asking for trouble.
In addition to tension you want to make sure the chain path is true between the engine drive sprocket and the rear wheel driven sprocket.
The tensioner bracket needs to be attached to the chain stay so it can not move or rotate inward into the spokes.
There are hours of reading here regarding that tensioner. Use the search feature at the top of the page, keyword, 'tensioner' and see what others have done.
Good luck.
Tom