I started out with 30 sec planks on front and sides. I slowly added 10-15 secs.
Now I do 2-3 minutes each side. I only do one set. I do side star planks. For my front plank, I hold it for 2 minutes then continue to hold it and raise one foot off the floor for 30 seconds and then switch up feet and hold for 30 seconds. It keeps me from being bored.
I do them 1 set 5x's a week for my physio. They are one of the few exercises that don't tear up muscle tissue so they can be done regularly.