I'm writing a Windows service using WinAPIs and C++. One of the options is that I need to send the system into sleep after all users are logged out. The log-out command is started from the UI (which can notify my service), the problem is knowing how long it takes for all user accounts to be completely logged out before putting the system to sleep.

I first thought to introduce an artificial delay from the moment when the log-out command is issued and before entering the sleep mode, but in a situation when some user account was configured with a roaming profile, the log-out process may well exceed my delay.

So any suggestions how to know if all user accounts are logged out "for sure"?