How many turns of a 737 trim wheel equal 2.5 degrees stabilizer deflection?
About 45 Rotations

In this YouTube video you can see Mentour pilot demonstrating manual trimming of a Boeing 737 NG. They start at about 11:30 with:


  we have 4 units nose down now


and at 12:30 they say:


  now we are at about 3 degrees


I counted 18 full rotations of the trim wheel in this time. This would result in

$$ \frac{18}{4^\circ - 3^\circ} \times 2.5^\circ = 45 $$

rotations for 2.5 degrees of trim change.

Note that the trim indicator is not very precise, so the number is only an estimate:


(frame from the video)