I have had ferns ranging from 5 days in a row to about 10 days.
Sometimes the ferns are there one day and gone the next. Sometimes I think one of the little ferny things must have slipped down the side so I try and clean it really really really well to be certain they can't hide as I don't get why they can be there one day and they go and then they come back again. Hmmm
Hope all goes well for you