Tough question. There certainly are people for whom therapy may need to be a lifelong commitment, but this is a rare exception. Generally speaking, therapy is not forever.
Have I succeeded in reassuring you?
Can therapy be brief - a matter of months rather than years? Yes, but that depends partly on the person seeking therapy. The difficulties people bring to therapy typically haven't arisen overnight. They're often years in the making and may be tightly intertwined with one's identity. I won't promise a quick fix.
That said, I acknowledge that many people are reluctant to sign up for an expensive, multiyear undertaking. Just because I know how deep therapy can go doesn't mean I believe it has to go as deep as possible, or last as long as possible, to make a difference.
Some people come to therapy with an openness to the idea that it could take a while. Such openness is very helpful. It acknowledges that problems in our mental and emotional lives are not as well defined as physical problems that can be treated fairly straightforwardly.
People leave therapy all the time before all the work is done. Sometimes they leave way too soon and for all the wrong reasons, but sometimes their departure makes a lot of sense. Feeling better, functioning better - these are real accomplishments.
I never assume that I'm the last therapist you'll ever work with, or that I should be. You may start the process of change with me, continue it years later with someone else, and really pull things together years later with yet another therapist. In my book, that counts as success.