I got my first Season 1 5* on a lucky EHT summons (Magni) at 10 months. Your mileage may vary. The rest started to trickle in after that, nearly all from the last level of the training camps (TC20).
Some of the player base determined the percentage of each hero type over time from the TC20 was:
5* = 5%
4* = 20%
3* = 75%
You can compare results on this thread:
Training Camp lvl 20 - Results - #3875 by Rook
As for HOTM’s or regular summons, I say it’s “luck (RNG) or money”. 