Typically expect that to be as long as it's a current product and staff aren't reassigned to newer products.
With Open Source you'll have access to the source, and you'll need to take ownership of that and maintenance of it and deviations to the reference design you introduce.
Longevity commitments tend to revolve around availability of ICs in production, not staff availability, continuity of employment, nor commitments to future kernel support and back porting.
Speak to your local sales rep or FAE
