15 min ? That is a warmup. Doing proper 3x5 squats take at least 35-40 min.
I'm in similar situation so I can give you 2 ideas that worked for me:
1. You do full-time workout whenever you can. 1 times a week? OK. 3 or 4 times a week ? Still good. You dont care when it is done and how much time seperate workouts. Just do it when you have time. This works good with 4 day split -
1. Squat volume + press/bench intensity
2. Deadlift intensity + press/bench volume
3. Sq int. + Pr/bp volume
4. Dl volume + pr/bp intensity.
This way you can handle 2 sessions in a row. Even 3 if the third is light pulling and 5rm press - no problem, I did this.
2. When you have time for full-time workout you do as much volume as you can. So basicly monday workout of Texas Method. When you dont have time you do only one lift as intensity day. Lets say you have friday off so:
Fri - 5x5 squat and press + deadlift
Mon - press/bench intensity + chins
Tu- squat intensity
Wed- press/bench intensity + chins
Hope this helps.