Normally the installment plan form (9465) requires you to say what you will pay, when day of the month the money can be pulled from your bank account etc. Best thing is to set up a personal account with the IRS and then you can see when the installment plan posts. IRS will probably send you a letter about the payment plan.
FWIW: Anything you pay online at the IRS tomorrow (4/18) will not accrue interest. The installment plan does accrue some interest.
