I figure on two years or so at least, but that's just going off of the Spanish Flu pandemic's length, which spread about as much as this has.
SARS and other more recent outbreaks are much more closely related to this case of Corona-Virus, and generally had a lot shorter length of duration than other outbreaks in history, but you can't really go off of that because those recent viruses didn't spread nearly as well or around the world as this has. Those, while often more deadly, were less contagious, and were far easier to contain and got stamped out within a short time when they disappeared.
Some things that could make this drag on longer than Spanish flu would be how much more traveling takes place now vs in 1918, plus back then people were often in rural areas and spread out, so they didn't interact as much, nor was there necessarily the need to. So with this I could say possibly 3-5 years even, if traveling continues on a wide scale. Plus people still have to go to the grocery store and other essential businesses in most cases, which spreads it more. In the past people were more self sufficient in a lot of areas of the country/world.
There are some promising vaccines out there in testing. Fortunately this is being pushed at a fast pace given how it's basically shut down much of the world. So hopefully at least one of those vaccines work. It would be even better if multiple ones worked and could be made at the same time, so even though it would be slow to make them, you could have multiples available and everyone that wanted to could get one or the other. Again, I'm not sure what the public's acceptance would be, and regardless vaccinating millions of people is a monumental task. But we really have to move on from this.
On the other hand, vaccines sometimes have limited success and there are a lot of people who wouldn't get it since they either never get them (I don't ever get the flu vaccine) or those conspiracy theorists who think it could be something sinister. Or others who wouldn't trust it's effects because it was developed in a rush. So whatever reason, it could still leave a large part of the world vulnerable, and we'd still have to social distance, masks, etc.
Aside from that all we have really is herd immunity which I think we are likely very, very far from. That's also very deadly given how we have practically no idea of how any person will fare if they get the virus. So it's certainly not the best route to take.
I'm hopeful but doubtful at the same time we could have wide-scale vaccines by next year hopefully. Still a long time but better than being shut down for two years or longer and being constantly in danger.
Now, if as some might believe, there is no immunity from antibodies, then most vaccines will not work. In that case this will keep circulating for many years possibly, and killing people. Although, I would think (hope) it would morph into something less harmful like other viruses have done, and eventually we could go out freely again without being much concerned about it. But that would take many years of isolating and masks, and would be indeed a terrible scenario. Not only for getting sick or dying, but the mental and financial toll would be grim.