Long term stocks/funds - How long exactly they mean when they say Long??

Long term stocks - How long exactly they mean when they say Long?? I am not a greedy fellow. Just that I am satisfied already investing in mutual funds, but also want to know if there are any long term stocks which I invest on, today and reap benefits when the time I retire? Also, how long we should really keep a particular mutual fund? Do mutual funds stop returning the same retuns after sometime and I keep reinvesting onto different ones?