Q: How do I estimate how long a format has left so I don't post into a dead trend?

Q: How do I estimate how long a format has left so I don't post into a dead trend?

Short answer: map where it is on the four-stage curve.

Longer answer, identify the stage:
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Stage 1 (Niche): only big creators in one vertical use it. ~3+ weeks left — best entry.
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Stage 2 (Crossover): jumping niches, brands not in yet. ~1–2 weeks.
—
Stage 3 (Mainstream): brands, parody accounts, your aunt. Days left.
—
Stage 4 (Decay): 'is this still a thing?' posts appear. Over.

Check which accounts are using it RIGHT NOW to place it.

Verdict: enter at Stage 1–2, never Stage 3. When you see brands and parodies, the format isn't trending — it's finishing.