How much time should be allotted to plan, design and budget for a kitchen project?

Is it best to negotiate for the contractor to handle all material including finish materials? What if any would be the savings in homeowner handling purchasing all finish material?

If you can take your time, do everything step by step there are so many good and affordable products and they are easy to install but be patient somethings do need prof. touch…

It is better for the project and client to select and pay for plumbing fixtures, appliances and light fixtures. Direct clients to vendors you have pre negotiated pricing with. Most vendors will assign an account rep to track your clients order and handle present and future issues with purchased items. Additionally in many cases vendors will provide free or low cost design consultation.

Typically there would be no savings in my experience, unless you or someone you know works at a supply house and can obtain a discount.
Most contractors have relationships and get discounts roughly equal to the markup they charge you.
And any markup is money well spent. It takes a lot of time to track down the materials, purchase and deliver them.
I allow customers to supply their own materials on occasion, but I bill for down time if materials are short or incorrect, so if you come up short on flooring, you would be paying my flooring crew while they wait for materials. If we have to leave the job and come back later there is a stoppage fee. This is because downtime kills our profit. If I have to pay a crew for half a day of non work, there goes my profit on installing the floor.
In almost all instances it is much easier and more cost effective to let your contractor purchase the materials. If something goes wrong, they are the ones jumping through hoops, not you. The little savings you might get by purchasing materials yourself is not worth the aggravation.
Just my thoughts.

It is usually better for the General Contractor to supply all materials including finish materials. If a homeowner supplies materials, they are responsible for incorrect color/style, shortages, damage during shipment and delivery, and warranty claims. Delays and frustration can occur for both the homeowner and contactor if the proper type and amount of material is not ordered and received on time.