The threshold of conscious experience is about 200 milliseconds, or one fifth of a second. This is enough time to pronounce a syllable.
But a syllable is not a unit of meaning. A meaningful thought can take several syllables. If the “psychological present” is 3 seconds long, the longest thought that is articulated subvocally “as it unfolds” is 15 syllables (assuming no pauses between syllables). Otherwise, the thought is being remembered. Can we remember thoughts while continuing to think?
(Here is an example of exactly 15 syllables.)
The added complication is that “meaning” is not cut-and-dried – meaning emerges and is transformed in time. A “meaningful” thought – that is, a thought that “says something” - doesn’t have a clear beginning and end, because what it “says” - its meaning - keeps changing in time.
In other words, thoughts are not countable units.