Nagabhushan, I do agree with Steven , you would need to use/implement logging.

it is possible to monitor users currently logged in

Yes you can

in ASDM goto

Home/Configuration/Monitor/VPN/Session

but what I want to monitor is users who were logged in since the firewall started (for example, 30 days)..

Since you don't have any permanent logging setup to send logs to a syslog server for that amount of time you have already lost that information from 30 days ago, you need to implement syslog .

Go over this link for monitoring, you can also filter syslog messages based on log ID number or logging severity levels

http://www.cisco.com/en/US/docs/security/asa/asa80/configuration/guide/monitor.html