Some very sloppy math suggests around 4.3ms:
[50 x ((80us as worst case to lock next channel + (time for 2 bits at 300kbps))]
So, on average, maybe about half that time, or 2.15ms, for it to find it (doesn't account for cases where the transmitter changes channels back to one the receiver had already scanned though, so that the receiver might find nothing even if it searched all 50 channels in a row).
Anyhow, it's possibly the right order of magnitude.
Feel free to use a sharper pencil.
It does drive home the point, though, that frequency hopping probably isn't a great substitute for use in the typical scenario of ultra low power nodes (ones that 99.99% of the time wake up, listen briefly, and then just go back to sleep if they don't hear anything).
[Edit: Nonetheless, for high throughput scenarios and/or scenarios where power consumption is of less concern and/or for higher security and/or scenarios where really long range can't otherwise be obtained, frequency hopping does seem to offer some unique advantages. ]