AD password policy does not follow the LDAP RFC, but Microsoft has implemented its own policy.
LemonLDAP::NG implements partially the policy:
when pwdLastSet = 0 in the user entry, it means that password has been reset, and a form is presented to the user for him to change his password.
when computed virtual attribute 'msDS-User-Account-Control-Computed' as 6th flag set to 8, the password is considered expired. (support from Windows Server 2003) It is too late for the user to do anything. He must contact his administrator.
a warning before password expiration is possible in AD, but only in GPO (Computer Configuration\Windows Settings\Local Policies\Security Options under Interactive Logon: Prompt user to change password before expiration) However it as no reality in LDAP referential. A "password warning time before password expiration" variable can be specified in LemonLDAP::NG to do so.
Note: since AD 2012, each user can have a specific password expiration policy. Then, the "maximum password age" can have different values. This is currently unsupported in LemonLDAP::NG because every policy must be computed with their precedence to know which maximum password age to apply.
To configure warning before password expiration, you must set two variables in Active Directory parameters in Manager:
Password expire warning : number of seconds between password expiration and the date from which user is warned his password will expire.
Password max age : number of seconds after the last password change, before it expires. It must match AD policy