There's an argument herd immunity can negate that but until there is a scientific study that confirms that people who have caught the virus retain immunity for a significant amount of time then you can't take that chance. (by scientific study I mean a peer reviewed, published in a reputable journal and verified, and not something some government minister or president says)
I would suggest the opposite is true, science would expect at least some period of immunity, as long as antibodies are created, and this is already a known fact for covid, as it can and is being measured with antigen testing. So unless proven otherwise in a peer reviewed study, like with almost every other virus, some level / period of immunity is presumed.
There are also studies into immunity prepublished currently being peer reviewed, such as this one
https://www.medrxiv.org/content/10.1101/2020.04.30.20085613v1 here’s a quote from the conclusion “232: Reassuringly, we found that almost all participants with confirmed SARS-CoV-2 infection in our 233: study mounted an IgG immune response to this disease.”
That being said we generally don’t need a peer reviewed study either way to know. The fact that no reinfections have been ‘confirmed’ globally, just a handful of anecdotal cases, said to be testing errors. Sweden has been studying this, and whilst there’s nothing (I’m aware of) published or peer reviewed from Sweden, do you honestly think they’d hang the lives of their entire nation on it if they were not sure?
On top of this, the NHS is actively treating patients with other people’s anti bodies in a clinical convalescent plasma trial. I don’t think they’d be doing this if all of those antibodies, we’re now able to test for, were useless ?
https://www.nhsbt.nhs.uk/how-you-can-help/convalescent-plasma-clinical-trial/
The real point to your question is... Do these antibodies last long enough to prevent a significant second wave in the winter, if the virus turns out to have some seasonality to it??? It’s likely to be the case, but a long way from being a given, and so is a huge concern.
Studies into SARS and MERS showed that immunity peaked after months, and waned slowly after that. So there is hope that it will last long enough to avert a large second wave in winter, in countries with high infection rates. But it’s not at all confirmed, and still a huge concern requiring some social distancing measures to continue. It is also ‘hoped’ that social distancing will reduce the spread of other seasonal illnesses limiting their drain on health services and saving capacity for any potential second wave.
Edit: sorry reading that back it sounds like I am focused too much on the fact that immunity is now all but confirmed, and not enough on your concern of ‘for how long.’ the reason for that was because the subject was lifting the lockdown... Whilst we do currently have some immunity, lifting the lockdown is not going to affect the R0 now, as much as not being in lockdown when when that immunity starts to wane, probably in or after this winter. So it’s absolutely the right time to slowly and carefully start injecting some life back into the economy... But without a vaccine, or a miracle disappearance of the virus, brace yourself for further measures from winter onwards...