? x heal points = uber?

How many heal points are needed to charge an uber?

It isn't a set amount of HP. It is based on time+hp. Meaning if you ARE healing people that aren't fully buffed, it will charge faster. But if you constantly heal someone even after they are fully buffed, your meter will still fill up.

An estimate is about 1000 HP.  But many things can vary this.

The ÜberCharge meter will fill faster (2.5%/second) when healing injured teammates or teammates whose health is not at 150% (at 150%, 1.25%/second).

source: tf2 wiki

That matches the times I have read, being 40 seconds (2.5 x 40 = 100) while healing hurt people or during setup, and 80 seconds(1.25 x 80 = 100) if you are healing healthy people.

So yes, like people above me have said, its time, not health healed.