Not as much as he had expected

Natsu had realized that the energy used seemed to be tied to the woman's willingness to do these activities.

Getting a handjob from Lucy was something she normally wouldn't want to do, so it took a lot.

But this... Only seemed to double her hourly upkeep. Meaning, two energy per hour to keep this up. He could maintain this for like 30 hours if he wanted. Granted, doing anything more than just having her ACT this way may do some more damage...