You might (think) you know how long it will take you; we don't. We don't even know if you're being honest, or deliberately underestimating/overcharging for work. Because an hourly rate means you have every incentive to do so.
* A contractor who insists on an hourly rate for a project with unknown timeline does himself and the client the favor of killing the deal.
* A client who insists on a total-cost pricing for a project they need budget-certainty on does himself and the contractor the favor of killing the deal.