It depends on the company and efficiency of the motor,bit I believe the general guideline is they pull about 1.1 to 1.3 amps per ft.lb of torque. So a 36ft lb thrust trolling motor would pull somewhere in the realm of 44 amps at WOT. A 50 amp breaker would be right for the job.
*You can also calculate a rough idea of how many hours you would get out of your battery if running at WOT using this same information. A battery with a 100Ah reserve would last about 136 minutes at WOT with your 36ft lb thrust motor. (100 / 44 = 2.27... 2.27 * 60 = 136.36) Of course this is an estimate. If you used a multi-meter to determine exact amperage, you can get a better idea--if you we're ever curious. If you normally run your motor on speed setting 3 of 6, test the amperage draw on speed 3, then do the math--and that would give you a more applicable usage time.
The formula is "Batt Reserve / Amps Consumed per Hour = Amp Hours" then "Amp Hours x 60(minutes)" Remember, everything running off the battery will draw amperage, so if you have more then a trolling motor, you have to factor that in