They ran out of samples
Their test is destructive, they can't test the same sample twice. They planned out a number of samples to test for each time period and tested them. At the end of six weeks, they were done.
It doesn't really matter
The point of the paper is that HCV is infectious long-term on surfaces when not cleaned properly, indicating there is ...
The place to find this information would be the Methods section of the paper.
We spotted 33 µL of plasma spiked with HCVcc on the 24-well plates. They were either immediately tested for viable virus or stored at 4°C, 22°C, and 37°C for up to 6 weeks before testing.
They tested their samples for (up to) 6 weeks. At the time of the 6th week they still ...