Hello Andrew
From my experience of dealing with most of the major manufactures
back in the 1990's, There is no simple answer to your question. Different models will need different process which will lead to varing build times, and in addition, if you have ever been past one of the manufactrues production plants, you will almost always have seen fields of standing caravans. You cannot tell from the road side but in some cases these may be stored awaiting for final parts, or rework.
Another variable and significant factor is that the majority of manufactures still use labour payed by piece work. This does not lend its self to the maintenance of high quality workmanship, hence the relatively high levels of rework.
By all means ask the manufactures but I think they will refuse or avoid question.