We present a timing analysis of the 2009 outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508 , and a re-analysis of the 2007 outburst . The source shows a short recurrence time of only \sim 2 years between outbursts . Thanks to the approximately 2 year long baseline of data , we can constrain the magnetic field of the neutron star to be 0.4 \times 10 ^ { 8 } G \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } B \lower 2.15 pt \hbox { $ % \buildrel < \over { \sim } $ } 9 \times 10 ^ { 8 } G , which is within the range of typical accreting millisecond pulsars . The 2009 timing analysis allows us to put constraints on the accretion torque : the spin frequency derivative within the outburst has an upper limit of | \dot { \nu } | \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 3 \times 10 ^ { -13 } % Hz s ^ { -1 } at the 95 % confidence level . A study of pulse profiles and their evolution during the outburst is analyzed , suggesting a systematic change of shape that depends on the outburst phase .