Each additional merit will reduce the casting time by 30 seconds.If you cap it to 3 merits, for example, the recast will only be 3 minutes (Notably the same recast time as Repair).
This ability instantly swaps current HP and ignores MAX HP similar to the way Convert exchanges MP and HP. If your Automaton's current HP is above your MAX HP, you'll just get your HP refilled. Your Automaton, however, will drop to whatever HP you had, it will not simply donate HP to you. For example, if you have only 7 HP, your Automaton will get 7 HP.
The conversion is a true swap of current HP so there is no possibility of a negative value returning which would result in instant death.