Regarding turn-around time from contracting/deposit to delivery:
If you want a biggish, functional Dob fast and cheap... well we all know where you can order that, Now, and expect it to arrive Soon! And that's exactly what you will get: Big, functional and cheap... and worth it!
If you want a big, superior, custom Dob... well we all know the handful of reputable, established, specialty shops that can work with you. The mirrors will be commissioned and the structure agreed as to features and provisions. Ample communications thru the entire process, from build to installation, to 1st light. That will take a while, cost more... and worth it!
Either approach can be the Right One... for You!
At work (aerospace optics) we had a (soto voce) Motto that went: "Faster/Better/Cheaper - Pick and Two."
Indeed, it seemed to be as inexorable as a mathematical invariance theorem... Never Violated. Customers' Wish-Lists would demand all three... but they and we knew the reality only allowed two. Anytime you pushed on one, the others would rear up.
CONCLUSION: A soft agreement/target for 1-year delivery on a custom scope... quite reasonable. If you know the shop is hustling, stand back and let them work.
PS: I'll throw this one in, customers might want to consider. If I'm working with the best vendors, I ask them to use their judgment re' details, so they don't get slowed down by customer horning-in, second-guessing every nit of execution. I also offer, preemptively, to accept extension and cover added expense if technical issues arise... especially if I'm wanting something developmental or beyond... which for me is... Always! Tom