Forum Moderators: open
In general I look at a page and if it looks "spammy" to me, then I've overdone it.
If it looks good, and reads well, then it's right.
I don't think you can really overdo the non-visual factors such as keyword meta-tags. My understanding is Google ignores them anyway. Image-Alt tags should be a simple description, and again shouldn't look spammy (these are "visual" elements as far as a screen reader is concerned).
TJ