My Artscroll Selichos says, right after Shema Koleinu:
During the recitation of the וידוי, Confession, stand with head an body slightly bowed, in submissive contrition.
How long does one need to do that for? Does this standing continue until the end of Selichos, or just until a certain point?
The Mechaber says (507:3) that one should stand for Vidui, and the Rama there adds that the main part of Vidui is the phrase "אבל אנחנו חטאנו" "...but we have sinned". So it seems that this is the minimal requirement for bending.
See Mishnah Brurah commentary 12 that states that even though you should say all the paragraphs standing, it is suggested to bend over a bit. He also seems to imply that the paragraphs between Ashamnu and "al Chet" such as "Ata Razei Olam" are "additional", seeming to mean that they are not actually "vidui" - confessional paragraphs.
In most shuls I've been to, the bending is only for the actual "vidui" parts, i.e. the paragraphs of "Ashamnu" and "al Chet". Thus, during the paragraphs between the two, one does not need to bend over.
Aside - the vague instructions of telling you when to start but not when to end something are not unique to Art Scroll. I've seen a number of Machzorim say to open the Ark at a certain spot, but not say when to close it. Then a few pages later, at another paragraph it says to open it. It's already opened! In short, the machzor is not written like a "technical / user's manual", though many think that it is :-)
