We filed for an extension after about 3 months after her death, and sent with that a check for $245K.
A year later we filed the final form 706 which called for a refund of $68K, which they promptly sent us.
Several months later we were informed that the estate filing had been chosen for an audit. That process resulted in a year-long ordeal of getting things re-appraised, getting old bank statements and so on.
The audit resulted in a reduction of the overall estate value. They did nail it for "gift taxes from the 1970's" but otherwise was quite favorable.
The final refund check did actually include about $4K in interest that the IRS gave back to us... which surprised me.
They do pay interest if they held your refund more than 45 days. The interest starts on the return due date.
I’m glad you got your refund. That doesn’t always happen!