Please note that in the use I'm familiar with, it means "at least."
"A good 20, 25 years" means "At least 20 or maybe even more than 25 years."
How long did you live in Maine? Oh let's see now - a good 14, 15 years, I'd say.
If you want to say that the time period has been pleasant, the intonation is different. "It's been a GOOD 25 years with Acme Products, but I'm looking forward to retirement."
A: We lived in Maine 15 years.
B: Yes, but they were a GOOD 15 years, weren't they?