#1 as millenia of 1000 years
#2. two mayan-epochs of 2,562 years giving the long count of 5125
#3 as a descriptor e.g victorian, industrial, digital, roman etc
#4 10 hylozoic-epoch of 2,500 years aligned to 10 zodiacal periods + leap period of 922 = precession of 25,922 years where Aries / Pisces are amalgamated as are Virgo/Libra
#5 twelve zodiacal-epochs of 2,160 years = 25,920 years
As you would expect we go with option #4
where Pisces started on 550 BCE and Aquarius started on 1950 CE.
In deference to the mayan count we also take into consideration two overlap periods – 62.5 years before official start date and 62.5 years after it.