In great expectation,chapter 15,what becomes of Estella?

Pip visits Miss Havisham and learns that Estella has been sent abroad.

Nothing says it better than the text-

"I had been looking round - in fact, for Estella - and I stammered that I hoped she was well.

"Abroad," said Miss Havisham; "educating for a lady; far out of reach; prettier than ever; admired by all who see her. Do you feel that you have lost her?"

Estella has been sent abroad by Miss Havisham to further her training as a lady. Just as Pip is in London to learn to be a gentleman, Estella has gone way to continue her education arranged by Miss Havisham.