The IRS generally releases the ACH transactions for tax refunds on Thursday evenings. How long it takes to make it to your bank account depends upon your bank, not the IRS.
Some banks post ACH transactions in real time, 24 hours a day, 7 days a week. Those would generally be in your account at the beginning of the banking day on Friday.
Many banks still post ACH transactions with the end-of-day batch processing of all of the day's banking business. In that case your refund would be processed Friday evening and would generally be available to you at the beginning of the next banking day, normally on the following Monday unless it's a Federal holiday.
A few very small community banks and credit unions are still not on the Fedline system and/or process ACH transactions manually. In those cases it can take an additional 2 - 5 days for the money to find its way to your account.
I work for a bank holding company that owns about 20 banks. We also provide back-end banking services for about 20 small non-affiliated banks. The normal nightly Fedline run takes about 3 or 4 hours. Thursday's took nearly all night and the Friday evening crew worked until nearly 5:00 AM Saturday getting everything pushed through. They usually get off between midnight and 1:00 AM. If we hadn't put on a couple of extra folks they'd probably still be there making sure that everything posted properly.
As a side note, about 15% of the IRS transactions rejected, indicating that someone had fat-fingered their account number on the tax return or that the name on the account didn't match the name on the ACH transaction. When that happens the money goes back to the IRS. They will then issue a paper check in a week or two. Make SURE that you get the routing number and account number correct when you file! And make SURE that your name in on the bank account that your refund is going to!