How long does AI image generation take?

Could you elaborate on the time frame for AI image generation? Are there any factors that can influence the duration of the process? For instance, does the complexity of the image or the capabilities of the AI system play a role? Additionally, is there a way to optimize the process to reduce the time it takes? I'm curious about the average time it takes to generate an AI-generated image, and if there are any advancements being made to speed up the process.