If I understand you correctly, you're asking should you keep 8 players this offseason or 8 players in the 2009 offseason? As in right before the 2009 draft, you keep 8 players, or right before the 2010 draft you keep 8 players.
I definitely think you need to wait until BEFORE the 2010 draft to keep 8 players. If you were thinking AFTER 2010 draft, then that is far too long. You don't want to impact trades or draft choices that were made this year, but going into next year's draft, I see no drawback to telling people they need to plan to keep 8.
BTW, why are you making this choice? Have you considered the impacts? Keep in mind that the more keepers, the more stagnant the standings become and the more difficult it is for a team in last place to make up ground. Sometimes adding too many keepers destroys a league because the non competitive teams lose interest. I'm not telling you it's a bad choice, but I'm just suggesting you make sure it's the right choice. Plenty of successful leagues out there keep 15 players, but it takes a special group.