What Does a Realistic Timeline Look Like When Working with a Custom AI Development Company

[RemoteState]

Trying to set realistic expectations before we go further into planning and honestly the estimates I am getting are all over the place.
We got proposals from three different teams for what is essentially the same project scope, a custom recommendation engine integrated with our existing product. One custom AI development company quoted six weeks. Another quoted five months. Same project description, wildly different timelines. At this point I genuinely do not know which one is being realistic and which one is either rushing or padding numbers.
This is our first time hiring a custom AI development company in USA so we have no internal benchmark to compare these numbers against. Would really appreciate hearing from people who have actually lived through a project like this from start to finish.

What People Who Have Been Through This Told Me About Realistic Timelines
Reached out to a few founders in similar spaces and got some genuinely useful context on how custom AI development services timelines typically break down:

• A properly scoped discovery phase alone usually takes two to four weeks and any custom AI development company promising to skip straight to building is either overconfident or cutting corners
• Data preparation and cleaning almost always take longer than teams initially estimate, several people mentioned this eating up 30 to 40 percent of total project time
• If a custom AI development company in USA gives you a single fixed timeline without flagging dependencies on your side like data access or internal approvals that timeline is probably optimistic
• Custom AI development services that include proper testing and validation cycles will naturally run longer than ones that rush straight from build to deployment
• Ask for a timeline broken into phases with clear milestones rather than one big number because that makes it much easier to spot where delays are actually happening
• The five month quote we got included buffer time for iteration based on real user feedback which the six week quote did not mention at all

Where This Leaves Us
Starting to think the six week quote skipped some important steps rather than being genuinely faster. Leaning toward the more realistic timeline even though it is harder to sell internally. If anyone has gone through a custom AI development company engagement recently and can share how your actual timeline compared to what was originally promised that would help a lot right now.