The influence of pulse duration on proton acceleration using subpicosecond (30–300 fs), ultraintense (from 3.6×10^18 to 3.6×10^19 W/cm^2), constant energy (0.14 J) laser pulses is studied using two-dimensional simulations. The entire pulse duration is modeled so that during the rising edge of the pulse a preplasma can naturally expand from the target front and rear surfaces into vacuum, altering respectively laser absorption and electrostatic field generation. In this paper, we study this effect for two target profiles (sharp-edge profile and smooth density gradient at the front side) and we point out the existence of a weak optimum pulse duration for proton acceleration. For the different pulse durations we consider, we first show that the maximum proton energy variations are similar to those of the rear side electrostatic field amplitude. The energy variations, however, are smaller than expected from the field variations, and we explain this effect by characteristic proton acceleration time.