Lots of different views but I think the one thing most if not all would agree on is no vapourware or land grabbing.
I think it is a bit of a how long is a piece of string question. It depends on the model. If it is niche model like your KUA, that will probably not be repeated, or if it is not for a long time, then letting people know well in advance so that the orders can build is probably a good idea. Additionally a niche model is less likely to face competition so you can allow times for orders to build.
No doubt when you announce your TT gauge Fell, you will need to allow some time for the 7 people on the planet who will order one, to find out about it. Other models like the class 37, that is likely to be in the range indefinitely, can have less of a lead time.
However, it also depends if you are duplicating an existing model. This could lead to a reaction and dumping of lots of product from a competitor. Therefore a manufacture may wish to keep it under their hat until the very last minute. This is obviously why we have not heard about your class 66 yet.
It is not east estimating demand and there will always be surprises in demand both good and bad, it is definitely a black art.
PS. I know Accurascale are not doing a TT gauge Fell, KR Models are