The more new era upgrades you put on, the less effective they are - but they're never worthless. Due to the "additive efficiency" rather than "reduced time" nature of the new eras, the resulting time is, for each possible number of new eras:
0: standard
1: 1/2
2: 1/3
3: 1/4
4: 1/5
5: 1/6
6: 1/7...