There are rules being introduced against issuing certificates for more than about three years (you can still get five years certs at the moment, but not for much longer); this is more to limit the risk of private key compromise than it is to require people to revalidate. I don't find it too onerous to have to install a new cert every year or so (he says, with several certs currently expired).
Still, I'd suggest issuing certificates for the maximum allowed length, unless requested shorter. Scalable revocation is possible; follow best practices for encryption keys by keeping a separate offline copy of a signed revocation request.
Revocation for X509 certs is a very different matter to that of revoking PGP keys. For X509, the CA can revoke the cert unilaterally, or at the request of the subscriber without the need for the subject private key. In fact, I'm not aware of the existence of a subject-side revocation process that doesn't involve the cooperation of the CA.
