How long does something need to be megulah to qualify as such? what is source?
Tuvya Marcus, Jerusalem
The Mishnah (Terumos 8:4) and Gemara (Chulin 10a) address your question. Their answer is codified in the Tur (YD 116): the amount of time it takes for a snake to creep up from underneath the handle of the vessel and return to its hole.
This is, albeit, not a very practical measurement of time for us. The Aruch ha'Shulchan (YD 116, 3), however, says it is a Shiur Mu'at (a short time).
Gemar Chasimah Tovah,
Yonasan Sigler
This is not a Psak Halachah