HTC had previously said that the smartphone should really get updated until the end of 2015. It seems that HTC will be able to fulfill the promise, and thereby to provide a high update rate of at least parts of their devices.
Android 6.0 Marshmallow can come for the smartphone HTC One M8 in the next couple of weeks. It is reported by a trusted source leakage LlabTooFeR via Twitter. However, note that this date called for Europe. When you upgrade the smartphone will also receive a new version of proprietary shell Sense. Now it will be 7.0 (although Sense 7.0 planned to distribute even earlier, along with Android 5.1).