Let's just get this out in the open. The product development process is extremely long and don't let anyone tell you otherwise.
Everything you read about Zara, H&M and fast fashion, it's only half the story. There are products they can move from design to the store in weeks. But they must be products in which the materials are housed next to the factory and are extremely similar in construction to one already in store. So they essentially skip the sample process and move directly into production. If there are quality issues, they can easily absorb that cost. The products that fit with fast fashion are usually slight variations of previous products they have already made. Some designs are even sampled without the intent of producing for the upcoming season. So they already did the work, just much earlier.
If it's your first time, and depending on how complex your garment is, expect 6 to 12 months. What we've found is that for new designers, changes are made more than 3x as often to the original design. The variety of factors vary from changes in order quantity, garment construction, or branding idea. New designers are also the most susceptible to changes between samples that are complete reconstructions instead of iterations. We recommend budgeting a month for each iteration, with the expectation it'll take 3-5 itierations.
Make sure to work backwards from the date of your expected shipment. You want to space at least 3 months between your final production go-ahead and the actual day it'll land at the port or at your door. So if you're looking to deliver by July, the order should have been finalized by April. In March, you'll want to have seen and approved your size samples. In February, the most important thing is to approve the fabric and construction on at least one of the sizes. By January, your bill of materials should be completed and confirmed.