user13061: Is there an absolute limit the payment request expiry?
I can't find this in the BOLTs, but maybe this is up to the implementations?

user81940: I think it is not explicitly specified. But c-lightning uses for example an unsigned int 64. Overall the expiry is obviously created to allow nodes to not store payment preimages for requested invoices for too much time. So therefor it makes sense to use values that don't extend too much actual time. I don't see why an invoice should for example be valid for a year. But technically it is certainly possible.

13061: <p>Is there an absolute limit the payment request expiry?</p>
<p>I can't find this in the <a href="https://github.com/lightning/bolts/blob/master/11-payment-encoding.md#tagged-fields" rel="nofollow noreferrer">BOLTs</a>, but maybe this is up to the implementations?</p>
 81940: <p>I think it is not explicitly specified. But <a href="https://github.com/ElementsProject/lightning/blob/01e5f1886e31816e652f417a1ff789a26aaeec3b/common/bolt11.h#L59" rel="nofollow noreferrer">c-lightning uses for example an unsigned int 64</a>. Overall the expiry is obviously created to allow nodes to not store payment preimages for requested invoices for too much time. So therefor it makes sense to use values that don't extend too much actual time. I don't see why an invoice should for example be valid for a year. But technically it is certainly possible.</p>
