Caching of Charts service images are based on the interval of the chart requested. Break down is

interval From 1 To 1 minute expiration is "00:00:20"

interval From 2 To 2 minute expiration is "00:00:40"

interval From 3 To 5 minute expiration is "00:01:00"

interval From 6 To 10 minute expiration is "00:02:00"

if no interval is specified then the default is 5 minutes. Settings are Charts service wide so cache timings cannot be changed/extended etc on the client side request.