This is a very common question being asked by patients struggling with depression, and understandably so. However... there is a hint of misunderstanding behind this question, in the first place. You see, depression is a complex condition with many concurrent causes, and to overcome the condition you will usually have to change those causes. How long that takes is entirely up to you, but there's a good chance it won't be an overnight process. That's now how it works, sorry... in fact, if would be better for you to simply not worry about the time it will take, and to simply stay focused on the process.
Not Getting Treated for Depression Takes Much Longer
Since you're concerned about the timing of a possible cure, you should consider the alternative scenario: what if you were to choose not to get treatment for your depression? Then it could worsen and linger for years, or even for a lifetime. Once you start getting treated, several seasons could pass until you're completely recovered -- but it won't take long at all before you start feeling so much better.
You may very well think of your treatment for depression as a long journey leading to a place where you'll feel happy and ready to embrace reality; maybe it could take months (or years) until you get there, but once you've started on this journey, life will get a little sweeter every day. Because you know what's waiting for you when you reach your destination: contentment and self-realization.
As you thread the ropes of recovery, you'll go from being caught up in a negative cycle of depression, to ascending towards a new, positive cycle in your life. With that in mind, it's actually better to stay focused on the actual course of treatment, rather than worry about how far you've come and how long it'll take to get where you want to be.
The Reasonable Time-frames for Anxiety Management
Having said the above, we should point out for the sake of clarity: depending on how severe your depression is and which treatment methods you pursue, it can take a full year until you feel absolutely depression-free and renewed as a human individual.
But the good news is that it will likely only be a matter of weeks after you start treatment, until you start feeling so much better than you are now. In fact, just deciding to get up and find treatment will make a huge difference in you emotions, shifting you from a victim's mindset to that of someone who's in charge of their fate.
Why not make this choice at the earliest, and be unconcerned with how long it will take?