The duration of the normal mode I actually choose after a playtest of a friend on an endless mode, but apparently I choose someone too good at top-down shooters because everyone I saw playing after it had the same problem to finish it. Will definetly test more the duration for it.
The general golden rule of thumb for arcade games that I've found is take whatever performance your average play tester had, and put on a 20-30% penalty on their score, then adjust the game accordingly until they've reached their original score.