When doing a plank should you hold it for an extended period of time e.g. 1 minute and do only 2 or 3 reps or hold it for 20 seconds do 5-10 reps and 4-8 sets?

When I go planks/bridges I do either 3 sets of 60-80 seconds or if I'm really wanting to hurt it, 2-3 sets til failure (hold til you collapse). IMO it's better to do fewer sets with longer hold times.

I do a bit of core workouts for my running training. These have helped me.
Core Stability Challenge- Bridge for 1 minute. Then take one hand away for 15 seconds, then swap hands for another 15 seconds. Repeat this with your feet. Following this, you do an opposite bridge for 15 seconds each side (take your right arm away and your left foot) and then change. Finish with a 30 second normal bridge. Once you've mastered that, do it on a fit ball.

'Monster'- A monster is made up of lots of different exercises designed to help your core. Things like crunches, cycling legs, tap outs (holding a bridge whlst using your arms to reach out to something in front of you) etc. If you start with 6 sets, you do 6 reps of each exercise. The next set you do 5 reps of every exercise. The next set you do 4 and so on.