The above diagram is provided from D.I.D. Chain, showing what the recommended chain slack should be on most off-road and street motorcycles. The slack should be measured with the swing arm parallel to the ground with the center of the sprockets in-line (as pictured)

Lubing your chain every 300 to 500 miles is recommended by every chain manufacturer. Before lubing your chain it is recommended that you ride the bike, clean the chain to remove any dirt, and then apply the lube. Allow the bike to sit for a few hours to allow the lube to penetrate and dry.

The more the chain is correctly lubed and kept properly adjusted, the longer it will last.