Generally speaking, no, I wouldn't consider that to be correct. You can say things such as "It has been a long time" or "It has been a long time since I saw her" or "I saw her a long time ago", for example. Do you have additional context, or are you just trying to write individual sentences, Frostwhite?
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frostwhiteIs this sentence correct?No. a long time ago is "definite time" grammatically even though the exact meaning in terms of a point in time is somewhat vague. Present perfect tense can't be used with definite time. Don't use it with any expression ending in ago.
It has been a long time ago.