There is no standard. It is what both parties agree to. Generally, the contractor will ask for an initial deposit to secure the customers scheduled start date (if not starting right away or at least within a couple days) The reason for this is that if the contractor and homeowner agree to start on a date, say a week or two later, the contractor may turn down another job or schedule the other job for a later date after the job in question. Should the homeowner decide to withdraw prior, the contractor has lost a potential job that otherwise would have been available.
In addition, should the project require a significant dollar amount of special order materials (Roof trusses, flooring, cabinets........), the contracotor does not want to get stuck with them as well as out of pocket expense should the homeowner default. Therefore, the contractor may require these materials be paid for up front prior to ordering.
On larger projects, the contractor may ask for a draw schedule. Here, the homeowner makes payments as the work progresses. This may be represented as a percentage or dollar value. This draw schedule should be detailed as to exactly what work is to be completed and how much and when the next draw is due. Again, this draw schedule may be required to purchase materials and cover labor as the job progresses. Otherwise, leveraging the contractors money would come at a cost.
It is recommended not to pay the entire job up front. A significant portion of the money should be held by the homeowner until the project is completed to his/her satisfaction and according to the original written agreement.
Generally, for me and my company, if the job is small with no special order materials, I may carry the job through to completion with only the full value paid at the end.
Mid sized projects (typical/average kitchen or bath) require an initial deposit of 30-50% and the balance at time of completion.
Larger projects like a whole house renovation/remodel or addition would be broken down into smaller percentages as the job progresses. This may be broken down into 3 to 6 draws.
Get to know your contractor. Go with your gut feeling and get EVERYTHING in WRITTING!!!. In this way, everyone is on the same page and each knows what to expect from each other.
Do your homework. Get references. Get second and third opinions. Keep in mind though the cheapest, is not always the best. General rule of thumb is to pick your favorite from the list of contractors that came in close to the same terms as each other. If the terms you get from various contractors is all over the place, get some more until you have some that are comparable to each other and pick from there.
Remember, you are the boss. You are the customer. Fair is fair and both parties should be able to communicate with each other until an agreement is found that covers both parties fairly