From a v$active_session_history perspective, a new row is inserted for every active session during each sampling period. So if you decreased the sampling frequency from once per second to once every 2 seconds, you would notice a 50% drop in the number of ASH rows. Assuming the size of the ASH buffer is fixed, decreasing the sample rate would also allow reporting further back in time.

ASH buffers are stored in the shared pool. Oracle wants to keep around 30 minutes of history, but the more active sessions, combined with the shared pool size and the sample frequency, will obviously influence how far back in time ASH activity is available. ASH space could be as little as 1MB or as much as 30MB.14 To see the current memory dedicated to ASH, execute the following:

You may be able to specify memory allocated to ASH by setting the _ash_size (in bytes) instance parameter, but experience has shown Oracle does not always respect this setting.