Common sense should dictate what’s too long. An adult out of college, in the workforce, earning decent money should be paying their own cell phone, insurance and putting a roof over their own head. Being in school, juggling minimum wage jobs, it makes more sense to stay home if your parents agree, and you are willing to live by the house rules. You can knock off a lot of student loans living at home for awhile after graduation, which makes adult life a lot easier.
I think 18 can be too soon for most to move out, especially if you’re not financially responsible. You can dig a financial hole that will take you years to get out of if you’re not careful.
By 25, you should be living on your own. Living at home when you’re not in school but working carries the responsibility to look after yourself, not create work for your parent(s) and make a contribution to the household. And you should be working towards a plan to live independently.