If you are offering advisory services only, you should propose a retainer structure, or a possible 50% retainer, 50% on milestones or completion. You could also do a monthly retainer based on the length of the project. If you are actually doing some type of implementation work, I would expect you could ask for a retainer/deposit and then bill the rest on time and materials.

How does your industry typically structure deals?
Will the customer focus on value-based billing or effort-based billing?
Are you managing a team of people?
What is your seniority and level of experience?

Without knowing more about the project, your experience, or the nature of the work, it's hard to answer this question but I would be happy to chat further, having many years of experience building and operating a professional services firm. There are many options around pricing models - the best fit depends on the project and the customer (and you!). Feel free to reach out to me directly if you would like to discuss further.