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 mark becomes (I) when the player does not login for another week. The name of the player also becomes grey when he/she becomes inactive (with a darker tone of grey being applied after 2 weeks of inactivity). In effect, the first week after the player is marked inactive his planets are marked with (i). The following 4 weeks his planets are marked with an (I), for a total of 5 weeks, or 35 days, of marked 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). Players inactive for more than 35 days are usually deleted, but not if they had some unspent Dark Matter remaining or a subscription for any officer(s) remaining. In this case their planets are frozen and will not generate any resources at all - not even the planet base production!
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.