We need to examine Grendel's ancestry in order to answer the question as to how long Grendel's mother has lived in the mere.
In lines 1260 to 1270, for example, the Beowulf poet implies that Cain is the father of Grendel: " since Cain had become/the sword-killer/of his only brother/. . .he went forth stained/. . .to dwell in the wastes./From that came many fateful spirits,/Grendel was one of them." Although the poet does not say in so many words that Grendel is Cain's son, the connection is inescapable.
If, then, Grendel is the son of the exiled Cain, and Cain fathered Grendel with his mother, it is reasonable to conclude that Grendel's mother has been in the mere essentially since the expulsion of Cain from Adam's family near the beginning of biblical time. Using conventional supernatural terms, we should probably conclude that Grendel's mother is an elemental earth spirit or, in this case, demon, having also lived in the mere since Cain killed his brother, damned by her relationship to Cain to live outside the world of men--"she who must inhabit/the cold streams/since Cain had become/the sword-killer/of his only brother."
Grendel's mother, of course, being a mother seeking vengeance for the death of her son, is a much more dangerous enemy to Beowulf than Grendel was and does much more damage to warriors and Heorot than Grendel did. When Beowulf realizes that he now has to deal with an even more powerful enemy than Grendel, he cannot be sure that his fight with Grendel's mother is going to end well for him. She has a tremendous advantage over Beowulf because she is fighting in territory she has known for thousands of years.