Except that you'll hit an issue with the automated renewal at some point and it'll likely be when you don't have someone available to deal with it - cue several hours of downtime. A problem could occur with the cert issuer and then you've got all of their customers with hours of downtime - not really a good idea.
90 days is a good compromise between encouraging autorenewal and allowing services to be down for a couple of days without really impacting anyone. It's short enough so that the person who set up the automation is probably still employed and thus they have an incentive to fix any issues.
Until it's too short that there's not a single worked day during the alerting period, I think it's fine. 45 days means 15 days between "it didn't renew on schedule" and "anything breaks".
90 days is a good compromise between encouraging autorenewal and allowing services to be down for a couple of days without really impacting anyone. It's short enough so that the person who set up the automation is probably still employed and thus they have an incentive to fix any issues.