I don't know that there is any hard limit. It would just be my instructor's judgment. But, we are both part of the same scuba "club" if you will, so I expect to be diving with him at least occasionally, outside of any class. So, he'll have a pretty good idea of how much I've been diving and whether I've been getting stale and need to do a full course.
So, yes, my current plan in effect is just wait for him to get the instructor cert and then get Helitrox.
My post was really to bring out the issue that I think the current TDI standards would make more sense if the Adv Nitrox and Helitrox certs as standalone courses did not have an in-water component.
Per your original few posts, after emailing TDI you did not believe there to be a time limit. This is what I understood to be the case. My mistake.
Regardless, again, per TDI, this is not an option anyway.
I don't have anymore information for you so I suggest you contact TDI directly if this is something you want to clarify with them yourself.