how fast does sierra overwrite over a permanently deleted file?

The OS, technically, has nothing to do with how fast something gets deleted; it's how quickly your drive implements the "erase" ATA command.

If you deleted a file by "moving" it into your Trash, it will be there until you empty the trash. However, if you deleted the file by using a Terminal command (rm for example), it's gone as you have bypassed the Trash altogether.

Now, if you are using a hard disk drive, there is a chance of recovery; the sooner you attempt recovery, the better the chances. However, if you are using a solid state drive (SSD), it's gone for good. The TRIM command to erase happens almost instantaneously; there's no remnants to recover because the block(s) that your data occupied are immediately marked as "new and unused" and the SSD is instructed to return zeros or random values if you attempt to read from it/them. See this answer for additional details on this process.