i've always been reccomended 2 and been fine with 2? i'm not arguing it just stating what i know
2 weeks is not long enough.
Read this its interesting but long.
<a href="http
/www.exoticpetvet.net/aqua/parasites.html" target="_blank">http
/www.exoticpetvet.net/aqua/parasites.html</a>
Taken from the above link.
Another common cause of a parasitic outbreak is inadequate quarantine of new fish, although many aquarists are loathe to quarantine new fish prior to introducing them into their permanent home. Many of the common and more serious diseases do not show up for two to three weeks following transport, and unfortunately, this can result in serious problems being introduced into the established aquarium. A quarantine tank can be a simple as a bare aquarium with a heater and sponge filter. Three to six weeks of quarantine should allow owners to identify any sick fish prior to introducing them into the community tank. During quarantine, weekly treatments with formalin (25 mg/L) are reasonable and likely to avert serious illness in many cases.