Simple question regarding boil times

Simple and dumb question, but some clarification would be helpful.

Example: When you have 2 LME additions during a boil, one at 60 and one at 15. So for the 15 minute addition, you take the pot off the heat, add the LME, and return to the heat. When does the final 15 minutes start again? Does it start when you return it to heat (~180F) or when it starts to boil again (~212F)?

I would think it starts again when it returns to a boil.

It's actually boil time. You're boiling the wort for hops utilization, and the hops oils are only isomerized when the wort is boiling.

If I was doing it, though, I'd just add the LME at the very end of the boil- like with 1 minute left. Then you can just finish up without waiting for the wort to return to a boil. The LME doesn't have to boil at all, but some people like to do it to ensure it's sanitized. Even adding it at flame out is perfectly fine- it's over pasteurization temperatures.

If your doing a full boil then so you really need to wait till the end to add the extract?

I do full boils and I always add all the extract at the begining and boil for 60 min. Works great.