I would say that after many tries the suction strength that doesn’t give you pain and is still making your nipple change shades to darker, means you start getting right stimulation.
But it maybe is best starting with slower to build the resistance to it. I usually start with slower one and then keep increasing as I pump to keep getting used to it, each time it feels normal and no pain and not too much pressure, press an up button again. I usually go from medium to max within initial 5 min. But, I do pump at least 1x a month for few days in a row, just so to make sure my nippples are used to it. But I don’t pump regularly anymore or trying to induce until I am in a committed relationship. I do lactate already a little (just few drops if I express by hand at any time) so for me maybe the pumping isn’t as important as for someone in the active lactation journey.
So, just pump up to the strength and speed that you enjoy and make sure that the frequency is more important than length of each session. So better you start 15min a session but 8-12 times of pumping to induce than 4-6 times pumping for 30 min each session.