Offtopic: I had to laugh at my dentist's text reminder of my time and date, with a line saying that late attendance would incur a £1 per minute fee.
I know it's supposed to be a deterrent, but if the terms worked both ways and the patient charged the business £1 a minute late, they'd be bankrupt within the month!
I do understand that it's the minority that ruin it for the rest of folk though, especially in a model like the UK's NHS provision.
They will if it's more than 45 days late, when that starts is usually going to be 'from tax day', that can vary, but it's usually for reasons that would typically be your fault (filed late, etc). They even use the same rates they would charge you if you were late to pay them. This has never happened to me, but I'm told that the interest accrual is automatic, supposedly don't even have to ask.
I wouldn't be surprised if the reason they are waiting forever for their tax return is because they didn't successfully exploit a loophole and got put into the pile for closer inspection. It's not like you just randomly have to wait longer than everyone else. Something happened.
Though the IRS does generally issue within a certain number of days from when you file (21, I think?), they aren't late until something like 45 days from the due date (April 15th unless you got an extension).