Outside Fact#1: Sethians were well-established and known in the time of Josephus, ergo 3 generations at least (before 10 BC). However, nothing suggests to me this
Untitled Text of Bruce Codex is any earlier than 150-200 AD. The text was found in Egypt and - since Xian themes permeate and overwhelm Egyptian Gnostic literature after c.300 AD - we might expect it dates closer to 200 AD than +275 AD. Uncertain. I suspect this material might be too overworked, interpolated to clearly date 'levels' of the text.
Outside Fact#2: Epiphanius'
Panarion (375 AD) describes Marsanes as long dead and legendary (2-3 generations). But Porphyry (c.260 AD) makes no mention of him, so it's likely they were contemporaries (even if P. didnt know of M. in this context.)
REVISION: Plotinus (205-270 AD) probably argued against teachings of Nikotheus' SCHOOL (c.250 AD): N., who was older & named by Porphyry. Porphyry's point was that Plotinus despised sectaries who read Nikotheus; Marsanes was N.'s student. I assume M. lived from c.220-270 AD.
I would therefore
date the terminus 'Untitled Text of Bruce Codex' c.270-280 AD. Which seems LATE, I think. ???
The core material may well date c.225-250 AD. In other words, THIS is the teaching which Plotinus despised; it may come from Coastal Syria. I havent read
these papers, however.
A few thoughts:
1) Marsanes was likely dead when this was composed, although he should have been alive recently: a decade or so prior.
2) Nikotheus was deceased: of the same sect, forebearer, a generation before.
3) Apparently, Post-Sethian; Setheus feels like (Late) Sethianism divorced from Judaism, w/ very little 'Jesus' attached.
4) Post-Valentinian, but caveat: much of the Gnostic material is indiscriminately labelled 'Valentinian'.
5) Hermes Trismegistus is name-checked but this doesnt feel Hermetic at all. (Hermeticism was dead OR this originates from an uninfluenced, distant cult.)
6) Where Theodorus of Asine (275-360 AD) may echo Marsanes c.270 AD, again a scholar is referencing material +60yrs older, ideas from one already long dead.
The powers of all the great aeons have given homage to the power which is in Marsanes (Marsianos). They said : "Who is this who has seen these things before his face, that he has thus revealed concerning him?" Nikotheus spoke concerning him; he saw that he was that one. He said: "The Father exists, surpassing every perfection. He has revealed the invisible, triple-powered, perfect one." Each of the perfect men saw him, they spoke of him, giving glory to him, each one in his own way. ... This is the only-begotten one hidden in the Setheus; this is he whom they called the light-darkness.