A long time can be expressed many different ways.
"I haven't seen you in a month of Sundays" is common in many parts of the USA. A Month of Sundays is actually a rather long time. If we accept there are 4 weeks, hence 4 Sundays, (on average) a month and and 30 days (on average) a month that would mean a month of sundays is 30 weeks or 7.5 months
"I haven't seen you in a coon's age" - meaning raccoon, has its origins in the South of the USA. Raccoons live up to 16 years in the wild but most die before reaching five years, so a 'coons age is about 5 years or more.
"...since Hector was a pup" is another phrase. It has rather more classic origins, dating back to just after the Trojan War. After wreaking vengeance on Polymestor for the murder of her son Polydorus, Hecuba (Hector's Mother) was transformed into a dog. Hector was a dog's son, *ergo,* he must have been born a pup.
"...since God was a boy." is rather obvious as God has been around a long time.
Thgere are many more, such as
"...in donkey years"
"till the cows come home" - etc.