After a general warm-up, say 5 mins on my C2 rower, I'll warm-up more specifically before work sets for each exercise. Anything involving shoulders, I do lots of light warm-up work. If I don't do this, my shoulders will hurt like crazy and keep me awake at night. I need to do less warm-up for lower-body exercises but, even during 15s, I'll do three warm-up sets of squats or deads before my work sets—these will often be for sets of 5 reps which get the blood flowing but don't tax my CNS much. Let the poundages you are using for your work sets guide you in how many warm-up sets to do. During 5s, it's ok to do doubles or singles for warm-ups as you get nearer to your working loads.
Eg. If, during 15s, my work sets were to be 2 x 15 x 110kg for squats then I might do this:
10 x 60kg
5 x 80kg
5 x 100kg
then work sets
If, during 5s, my work sets were to be 3 x 5 x 160kg for squats then I might do this:
10 x 60kg
5 x 80kg
5 x 100kg
3 x 120kg
2-3 x 140kg
1-2 x 150kg
then work sets