Which CAs do that? I get alerts from our monitoring system 30d prior to expiration and have not had any problems generating new certs then (and they expire on the same day as the previous -- i.e., I don't "lose" three weeks if I renew three weeks early).

I'm using StartSSL for a personal site and it didn't let me generate a new one until it had almost expired. I'm using the free verification though, it might be different now or for their paid versions.

well 15 days, isnt that bad.

I've never heard of a CA doing that. By and large, the reason why this happens is human error - no one knows the cert is about to expire because it's not being monitored and/or the email from the CA (if they even send one) goes to a mailbox that's not being read.

But there's no reason why a rote task like renewing a certificate should be left to humans. It should be automated, which is what my startup, https://sslmate.com/, is doing.

you should put cert checker on top, seems like a great feature

If your CA doesn't let you renew your cert well in advance of expiry (which I find hard to believe), get a new cert on a different CA. You can have multiple certs valid for the same domain name, from different providers; the active cert is the one you serve.