What is the length of a DUPLO train bridge?
Based on the layouts from this answer, a half-bridge seems to be 2 straight tracks + 1 quarter-circle (split mid-track at the end, because the middle of the bridge is centered with the crossing track below it).

From this guy's measurements, a quarter-circle of the newer DUPLO tracks is about 18x18 studs. Straights are 8 studs long.

So, a half-bridge should be 32 studs (16+8+8), or 4 straight tracks. The full bridge would be 64 studs, or 8 straights.

Note: I don't have the pieces to confirm these measurements.

Note 2: I now have the pieces and can confirm these measurements, as shown in the picture below.