Assuming that you purchase a current model iPad, history demonstrates that Apple typically offer five to seven years of system software updates. Unlike Android devices, Apple only cease providing software updates when device hardware can no longer provide adequate performance with updated system software.
For iPad and iPadOS, Apple releases a new major version update each year - supplemented with regular feature and security “point” updates throughout the year.
Some mainstream third-party App Developers only support the current and preceding one or two major OS versions. As such, as long as you keep your device updated to the current version of iPadOS, you might reasonably anticipate having several years of App updates beyond end-of-support from Apple.
As such, for current models - and assuming that individual App Developers continue to support development and update of their individual Apps - you might reasonably anticipate a worst case of at least six years of third-party App update support. Some App Developers maintain App support for as long as is technically possible for a specific App to support multiple OS versions.