With Android, I believe the setting is not based on the age of the message. But, rather on the actual number of messages.
The maximum number of messages is set by opening the messaging app (NOT Handcent, but the stock messageing app) then menu / settings / text message limit (for SMS) / multimedia message limit (for MMS) note: also make sure you select "delete old messages" box to clear out the old messages once limits you've established are reached.
Since there appears to be no setting inside the Handcent app regarding the number of messages or timeframe to keep messages, I would assume it uses the stock settings to manage this.
Keep in mind you haver LOTS of memory on this device (as compared to blackberry) and you are able to back up SMS with some free apps in market.