From End of Software Support - Clarifications:

Another option available to all customers with legacy products is to take advantage of the Trade Up program, which allows you to upgrade older Sonos products to modern ones with a 30% discount. Trade Up will be open to customers at any time should they decide to upgrade. 

“any time” - so I have at least a year?

Even better, you have no limit on when you can trade them in.  The offer is indefinite.

Thanks for confirming.

Why is the Sonos Bridge not listed at upgrade-eligible?

I believe because it was sold at a low price or even included with early Sonos products.

I thought there was a Bridge to Boost for 30% off or about $70 deal though.

https://en.community.sonos.com/components-228996/sonos-are-suggesting-i-replace-my-bridge-with-boost-why-6829193/index2.html#post16373945

More likely because it’s not a speaker, with limited memory and CPU, it’s a network device. 

OMG -- People:  the upgrade program exists for as long as Sonos offers it until they don’t.  It could end tomorrow.  It could last till someone’s newborn grows up and wants to buy a Sonos Play5,000.  Or the program could have ended while you were reading this.

depending on how the divergence in systems emerges in reality, you might want to keep a bridge to communicate with older units.

It wouldnt be beyond anyones imagination that the idea of 2 parallel systems might even stretch to having a sonos net 1.0 with old components and a faster and more reliable sonos net 2.0 for the newer ones. 

Nobody knows! 

And they were given away for free with any purchase for a good portion of their lifespan, so many people have multiple units gathering dust.  Giving 30% off any Sonos item for Bridges people have stacked in their closets is not financially wise.