That depends on the drive having power loss protection, which comes most of the time in the form of a capacitor that powers the drive long enough to guarantee that its buffers are flushed to persistent storage.
Consumer SSDs often do not have that, so flushing is really important there, at least if your data, or no FS corruption is important to you.
Enterprise SSDs almost always have power loss protection, so there it isn't required for consistency’s sake, albeit in-flight data that didn't hit the block device yet is naturally not protected by that, most FS handle that fine by default though.
Note that Linux, for example, does by default a periodic flush every 30s independent of caching/flush settings, so that's normally the upper limit you'd lose, depending on the workload it can be still a relatively long time frame.