If someone got a cbc in february is it possible to have developed leukemia between then and now? how long does it take for leukemia to develop?

Possible: The leukemia was likely in process in february, but was not detectable in a cbc, and has advanced enough to be detectable now. The time of onset of tumors including leukemias is generally indeterminate, you can only pinpoint the time when the tumor/leukemia becomes detectable. The underlying process has usually been in play for years.

Yes: Essentially two types of leukemia: acute and chronic. If acute, yes it can develop in a few months. Chronic can come on rapidly, however, it may be that the person never had a CBC before. The difference is acute needs treatment on an urgent to emergent basis and potentially curable. Chronic is typically not curable. Hope this helps.