So we need to formalize this, obviously.

Method 1: Exponential discounting.

Problem: You don't care very much about future people.

Method 2: Taking the average over all time (specifically the limit as t goes to infinity of the integral of utility from 0 to t, divided by t)

Conclusion which may be problematic: If humanity does not live forever, nothing we do matters.

Caveat: Depending on our anthropics, we can argue that the universe is infinite in time or space with probability 1, in which case there are an infinite number of copies of humanity, and so we can always calculate the average. This seems like the right approach to me. (In general, using the same math for your ethics and your anthropics has nice consequences, like avoiding most versions of Pascal's Mugging.)