Hello; The incubation period for some diseases or parasites problems to show up can be several weeks. I use to keep fish in quarintine for two to four weeks, but after reading threads about some nasty parasites have gone to a longer period myself. If your friend has not added any new fish to a home tank for two months prior to you getting one then the risk should be reduced. I plan to use at least six weeks or more for fish from any source.