There are five levels for this ability. Two more levels may be unlocked via ability bonuses. Note that Healing Surge always costs 30% of your maximum Mana to cast, and up to 20% of your maximum Mana to sustain the charge. All levels also restore 20% of your maximum Health immediately upon cast and 10% on finishing the charge.
Since Healing Surge uses a percentage (up to 50%) of your maximum mana to cast, it is not wise to use this skill while under the influence of any mana fortifying effects, as this will cost more mana and take longer for your mana to regenerate back to full.
If your maximum mana is 500, healing surge with full charge will cost 250. If your mana regen is 1 point per second it will take 4.16667 min of actual time to compensate.