Believe it or not, there’s research (?!)
From the book “The House Fly, Disease Carrier” (sorry folks, out of print):
“Curiously enough, few exact observations have been made upon the frequency with which the fly deposits its excreta. Major N. Faichnie, previously referred to, working in India, found that when a fly is put in a clean paper box it passes its excrement fifty times in twenty-four hours; that is to say, about once every half hour; but he neglects to state whether there was food in the box. Presumably there was some food, and also presumably there was not much of a semi-liquid character. Cobb (1910) gives a table of the intervals between defecation of a well-fed fly, together with notes on the spores in the excreta. One naturally infers, from the title of the article, that the fly in question was a house fly, but upon consulting an important paper by the same author (1906), entitled “Fungous Maladies of the Sugar Cane,” the same table is found printed on page 64 and the fly in question is said to be a Sarcophagid, and therefore not Musca domcstica. In his opening paragraph in the 1910 article, Doctor Cobb explains, “In some of these paragraphs, however, the statements are inferences fully justified by experiments with very similar species,” and this table is evidently one of these inferential statements. It is not safe to state that because, as shown in the table, a well-fed Sarcophagid fly will defecate on the average once every four and one-half minutes, from half past nine until half past eleven, a true Musca will do the same. It is by no means impossible that it will do so, but unfortunately we have not the proof. Still with this explanation it will be interesting to state that in the interval between 9: 35 and 11:26 the fly observed by Doctor Cobb (it had been fed at 9: 23) made twenty-three fly-specks at intervals varying from one to fifteen minutes, an average of about four and one-half minutes.”
So every 30 seconds doesn’t sound accurate, no.