Provided are a method for performing wireless communication by a first device, and an apparatus for supporting same. The method may comprise: starting a timer for an active time; receiving, from a second device through a physical sidelink control channel (PSCCH) based on a first sidelink (SL) resource, a first sidelink control information (SCI) including information related to a time domain of a second SL resource and information related to a frequency domain of the second SL resource; receiving, from the second device through a physical sidelink shared channel (PSSCH) related to the PSCCH based on the first SL resource, a second SCI and data; determining a value of a SL discontinuous reception (DRX) hybrid automatic repeat request (HARQ) round trip time (RTT) timer, based on a time interval between the first SL resource and the second SL resource; and starting the SL DRX HARQ RTT timer.