Dr. Mundo resists the first Immobilizing effect that hits him, instead losing 3% of his current health and dropping a chemical cannister nearby for 7 seconds. Moving over it reduces this ability's cooldown by 15 seconds and restores 4% of his max health. Enemy champions moving over the cannister destroy it.
Dr. Mundo regenerates 0.4-2.5% max health every 5 seconds.
About building Mundo as a tank, he is extremely nice as OT (or even MT) and you don't build him as a tank because of the passive, but because of his ability to hurt a lot even without having a lot of gear that increases AD.
Often causes people to build Mundo like a tank however when he doesn't fit that role very well
This is per second though so its 1.5% per 5 seconds
1000 hp - 15 hp/5sec or 3 hp/sec
2000 hp - 30 hp/5sec or 6 hp/sec
3000 hp - 45 hp/5sec or 9 hp/sec