According to boeings information for the 737 there is no time limit at all on G/A thrust, but there is of course a time limit on TO thrust (in our case always 10 minutes). However, that applies only to unreduced TO thrust, if reduced TO thrust is used check against MCT and if it is higher apply the time limit. To make it operationally easier my company has decided to always apply the time limit, although thrust reduction is in all truth a thrust increase in more than 99% of all cases.