Five seasons at least, huh? That's one or two more seasons than I thought the premise could be stretched to, but now, I dunno, I think they can do it. Six should be the limit, tho. Want it to go out on a high note. The danger is being repetitive. I noticed it a little as of S2, so it's only going to get worse.
Deb should find out the truth about Dexter but not until the season finale cliffhanger going into the second to last season at the earliest. Maybe they should wait till the last season.