AFAIK PNRs are GDS specific not airline-specific.

That is why, for example, around the world tickets have multiple PNRs but not as many as the number of airlines on the ticket.

A ticket can be valid for up to 12 months after first flight, which can be up to 12 months after purchase. Thus a maximum validity of 24 months (just under).

Some GDS have restrictions on characters used in their PNRs. For example Air NZ has an in-house GDS which uses the letter "H" at the end of every PNR. Their PNRs are five alphanumeric + "H".