To reset your keychain in Mac OS X 10.4, Mac OS X 10.5, and Mac OS X 10.6 Snow Leopard or later: Open Keychain Access, which is in the Utilities folder within the Applications folder. From the Keychain Access menu, choose Preferences. Click General, then click Reset My Default Keychain.
Keychain Access User Guide
Microsoft Wants To Access Confidential Information Machine
Your keychain may be locked automatically if your computer has been inactive for a period of time or your user password and keychain password are out of sync.You can set a length of time that Keychain Access waits before automatically requiring you to enter your password again.
Redbooks
In the Keychain Access app on your Mac, click “login” in the Keychains list.
Choose Edit > Change Settings for Keychain “login.”
Select the “Lock after” checkbox, then enter a number of minutes.
If you want to require a password each time the computer goes to sleep, select the “Lock when sleeping” checkbox.
Click Save.