Currently in DCS the Straight-Line counter (SL) only appears when firing an AIM-7 in FLOOD mode (and even then, it doesn't change). The SL timer should only appear when firing a missile on an AOT target without a known range, such as when the target is jamming. Currently with a jammed target the range reads as 99.99 meaning the AIM-120s time to go is around 150 seconds. With an AOT file there should be no indicated range, and the timer in the HUD and RDR should be the SL timer, indicating how long until the missile reaches Rmax.
There is some more nuance to the SL timer and more info can be found in the 742-100.
AIM-7SLTest.trk
AIM-120SLTest.trk