Ah, got it. If you can log in to the shell, type 'last' and that'll give you a pretty good idea of the outage window. It'll show you when people have been logging into the server. lastlog usually also shows shutdowns and reboots.
Edit: However this only shows machine uptime. If it was just the Apache server that died, then only DreamHost knows. If you use a free (or paid) monitoring service, it may give you better information.
You may also look through your /logs directory for timestamps of the activity on your site.