An inactive account is marked after one week of inactivity.
If a player does not log in to OGame for one week, all the planets of that player are marked with a (i) symbol in the Galaxy View. The name of the player also becomes grey when he/she becomes inactive. In effect, the first week after the player is marked inactive his planets are marked with (i). After an additional 3 weeks of inactivity, the players planets are marked with an (I), for a total of 4 weeks, or 28 days, of inactivity.
Most of these player's planets still generate resources and may be a great potential for plundering resources, as long as they are not in Vacation Mode (in which case they cannot be attacked). In addition, players with Noob Protection may attack strong inactive players. Inactives are generally good for raiding, as you are a far less likely to be scanned back or get hit in revenge. Most people can make a profit by raiding inactives on a regular basis (unless of course everyone else is raiding the same inactive).
But be careful: Since a player with a Sensor Phalanx may be monitoring local inactives. If the inactive is in the same Alliance or on their Buddylist in an Alliance Combat System universe they can Ninja the attacking fleet. Or they can time their attack to land seconds after your attack returns, before you can save it.
Player accounts marked as inactive will be deleted after 35 days, but not if they had some unspent Dark Matter (paid for with real life money) remaining or a subscription for any officer(s) remaining.