I believe historically Nexus devices have received 2 OS updates in their life cycle, right?
Nexus One - shipped with Eclaire then got Froyo and ended on Gingerbread
Nexus S - shipped with Gingerbread then got ICS and (likely) ends on Jelly Bean
Galaxy Nexus - shipped with ICS then got Jelly Bean and (likely) ends on Key Lime Pie
I highly doubt the NS will get anything past JB (at least officially) considering its aged hardware.
As for the Galaxy Nexus, personally I think it will depend on the hardware of the Nexus 4 - if it's a quad-core then the GNex will likely stop at Key Lime Pie, but if the Nexus 4 is only a dual-core (which I doubt) then the GNex has a much better chance of getting an update past Key Lime Pie.
If it will work reasonably then we will get them. Much of the reason Google sold this direct was to have a virgin phone out there not shackled by any carrier or subject to the carrier's schedules and needs.
As far as updates go for any particular Nexus I hope Google will update nexus device as long as the hardware will support the update without issue plus utilize all added minor/major software improvements. Those improvement must be optimize and perform as well latested nexus running the same os. The Nexus S with Jellybean compared to Galaxy Nexus with JB the Nexus S performs slam or just minor notch below compared to the GN across the board. As long as an old nexus device benfit and performance difference negligible I see no set limit for updating nexus device as long as it meets the technical aspect. Now the buisness side of updating nexus device is another story.
If a Nexus device is no longer being sold would it be worth the companies time and money to work on a update for the device more than two years after it's launch? It depends on the device for example the Nexus One and S I see no point compared to the Nexus 7 or GN. The reason is the Nexus 7 and unlocked GN have no carrier interference to getting update two those devices would be either as well both sold through the play store with no contract. Not having contract would create more rollovers to new nexus devices coming out presumably yearly. Bargain customers would buy the one or two year old nexus device sold on play store because it would be cheaper than current but be running the same OS. Their are also the loyal customers who own nexus device bought from play storw for 2 to 4 years this where Google would make sure these devices stay updated because what do you really gain not being on contract with carrier if you don't get update to your nexus after two years if that was your prime reason for nexus device from the play store.
Ultimately GN and up Nexus devices get update for at least three years but really 4 years would be perfect.