Rush hour travel time from San Marcos, TX to south San Antonio?

Basically I will be commuting for 6 weeks from San Marcos to exit 150B (SW Military Dr.) in south San Antonio. I don't know how long it takes in rush hour and I am trying to figure out when I should leave trying to be there by 8 AM. Is 6 AM early enough?

Six should be plenty early. Most of the (morning) rush hour traffic is northbound. You're probably looking at just over an hour, maybe an hour and 20 minutes if it's really heavy.

The normally 20 minute drive from 1604 to Downtown turns into at least a 30 minute drive at 7 AM, student traffic heading to SAC (San Antonio College). I make it from 1604 to 150B in 25 minutes at 5:30 AM.

I think I would try starting at 5:45 that first morning and then see what needs to adjust from there. Also keep an eye on the rain and fog conditions (don't know if Weather Channel gives you Austin or San Antonio's weather).