Honestly, there is no rhyme or reason for the most part. Not every return will be the exact same. Some take longer to process and some don't take as long. As long as you don't see a notice/tax topic/code on the Where's My Refund page to contact the IRS or they haven't sent you a letter, you shouldn't worry.
Processing times vary from return to return. Just keep an eye out at Where's My Refund site for the latest status on your federal tax refund. They will reach out if they need more information from you or if there is an issue.
And if you are expecting a state refund, you can use your State Department of Revenue's website to check status of state tax refund. Choose your state here How do I track my state refund? and it will give you steps to track the status. State processing times vary from state to state. Some have noted they will take up to 90 days or longer to process returns and issue refunds. So be patient!