The factory can tell the dealer when the order is officially placed the approximate time when the unit will be produced. Those production runs are scheduled far in advance with a large manufacturer such as Jayco who runs off hundreds of units a month if not thousands. A few weeks after the order is placed they will know the exact date of production.
The variable is usually shipping from the factory to dealer. There are three ways that works. The dealer will have Jayco do the shipping and Jayco will use one of the independent transport companies. Depending on the industry at the time, that can result in delays. The dealer can contract with a transport company and then have more control over it, or some even send their own personnel to pick up new rigs. Expect at least two weeks at a minimum for shipping from production.
Personally I will never order another, I will pick from dealer stock where I can compare different units of the same floor plan. When the ordered unit arrives it's your unit, take it or leave it. If it happens to be the lemon then your in a pickle.
We received a Rockwood in Dec of 15 that was ordered in late Oct. It took about 6 weeks from order to delivery. The unit was junk and had many build issues, some big ones such as misaligned slides, evidence of water damage incurred during shipping, etc. Ultimately we rejected that particular unit and after much discussion with dealership GM and the factory rep, another was ordered late in Feb of 16 and was built early in April. It was delivered late April of 2016, had the build quality we expected and was purchased the end of that month. That whole process took 6 months.
Make sure your deposit is 100% refundable and get it in writing, signed by dealership management, not the salesman. They'll most likely not want to provide that. If you push hard enough they will.
Another thing is those production schedules have been known to change especially when all the orders start to come in from the RV shows held in Jan/Feb. If I had concerns about being ready for spring I would order sooner rather than later, when the money started flowing out would not be a factor.
Good luck.