https://learn.microsoft.com/en-us/azure/cognitive-services/custom-vision-service/faq
https://stackoverflow.com/questions/59624674/azure-custom-vision-choose-training-time
Upon the above information the training time for an Azure Custom Vision model is not directly tied to the number of images you're training on. It's more about the complexity of the model and the quality of the dataset, and the reserved budget in hours is something you set based on these consideration.
If you're using the API to train your model, the reservedBudgetInHours parameter allows you to specify the number of hours reserved as a budget for training