Till 40 sec my RAM preview is happening.
If my audio file is of,let's say of 3 min duration, RAM preview (NUM 0) is happening for only 40 sec.
After that again it starts from the beginning.
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That is expected behavior. See this page. Short answer: you don't have a lot of RAM and it is getting full. AE is not like a video editor; it works with completely accurate, totally uncompressed, full-frame pixel information. This takes up a lot of memory. If you want to see the whole project, do a render.
AE is meant for creating shots. Most shots are a few seconds long. Forty seconds is a long shot. Three minutes is ridiculously long for a shot. Now, I know you're not likely doing one shot for that long. It's probably a motion graphics piece like a visualization or something, but the point remains. If you want to see the whole thing, do a render.
If you want to preview from later in the piece, either adjust your work area or adjust your preview settings. What setting to adjust in your preview settings will depend on what version of After Effects you are using, but in any case, the preview panel should have options for you to start your preview from the current time rather than the start of the work area.