Some ferrets are able to hold and kill rabbits quicker than others.
I tend to use medium sized hobs which are able to make kills pretty quickly and once they’ve made contact with a rabbit it doesn’t go too far.
Smaller ferrets can struggle with a large rabbit and will get dragged about once they are latched on to one.
The answer to your question generally depends on the size of the ferret you are using, and how experienced it is.
With my more experienced hobs, if nothing has bolted – or the ferret hasn’t reappeared within five minutes – I would put money on it having killed underground, or having got one in a stop end.
The only other factor that can make a difference is the burrow; if it’s really deep and consists of many tunnels then a ferret will need time to work the system properly.
However it is easy to see what the ferret is doing by tracking it with a locator – if it stays in the same spot for some time and you’re happy it’s not moving – start digging.