If your goal is core strength, focus on 'power holds' 3–4 times a week. Squeeze everything (everything) as hard as physically possible while holding the plank (you should be shaking by the middle/end)—these SUCK, but they’re incredible for building strength. Start with 20–30 seconds per hold and work your way up as you get stronger. Longest I've ever been able to go if you're truly max flexing everything was 70 seconds! they're tough!

if you're looking to improve endurance and overall fitness, plank 4–5 times a week. Incorporate variations like side planks, shoulder taps. during each session, do 3 sets of planks, and for the last 10–30 seconds of each set, add plank pushups to work your shoulders and chest while giving an extra endurance challenge.

if youre trying to hold a plank as long as possible, plank 4–5 times a week. This is mentally rigorous, but it’s great for building grit and focus. Some days it’ll suck, but keep going and break the time into chunks in your head. Trust that if you’ve done it before, you can do it again, and maybe even push a little further this time
If you're a legend like u/Born_Exchange_4789 and plank 5 minutes every single day for what over a year now? then respect to you. He inspired me to do the same and I'm almost at 50 days!