Under Manage Affiliates you have the option to set up an Automated Affiliate Threshold. You can enable auto-approval based on the Affiliates sales stats by: number of sales, sales amount by dollar and refund rate less than a certain percent. Once you have enabled this feature, any affiliate who requests to promote your product and meets your threshold will be automatically approved.
You can locate this under the Sellers > Manage Affiliates.
To set up your Automated Affiliate Threshold, watch here: