How long can I update my iPad M1?

Welcome!!

As we are not Apple, we cannot say officially. You can estimate by looking up a product history with the free (and excellent) MacTracker app:

https://apps.apple.com/us/app/mactracker/id430255202?mt=12

I looked up the iPad 8 (intro: 2020) in MacTracker. Its original iPadOS version was 14, and it topped out at v26, so it made it through six iPadOS versions. In addition, Apple historically provides functional and security updates for the current OS version and two prior ones, so add two years. In special cases, Apple have released security updates for devices that are outside the generally observed parameters.

So no one knows. It's like a "how high is 'high'" question. Some hardware differences between A-series processors and M-Series processors could change the timing in that progression. In my mind, an old iPad is still useful until apps and sites I use no longer recognize it.

While Apple does not pre-announce end-of-support or end-of-life for its products, for current models of iPad and iPhone, history demonstrates that devices are typically fully supported for both software updates and hardware repairs for 5-7 years from their initial release to market - or perhaps a little longer.

By example, the fifth generation iPad Pro (M1) was released in April 2021. As such, you might reasonably anticipate that software support will continue to be available for a few more years - although within the latter years when mainstream support ends, might be restricted to point security updates.

Announcements concerning end-of-life usually coincide with release of a new major version of iOS/iPadOS - major release updates historically occurring each autumn. Each year, some of the very oldest devices may lose support.

No iOS/iPadOS devices have a fixed support lifespan. Update support is only lost when a software update exceeds the system hardware capabilities of the host device - most usually being characterised by insufficient RAM to run the newer OS version with adequate system performance.