Does a CA require a full-time administrator?

I do PKI architecture as my day job, the answer is that it depends on the size of your organisation, the PKI you intend to implement and how well the system and your internal monitoring is set up.

If your organisation is large then there will be a lot of machines with certificates, these certificates expire, need replacing etc, however my experience is once you deploy certificates then any failure of a machine to communicate with another one is blamed on the certs before even the most basic diagnostics are performed. Ive seen certs blamed only to find the network cable unplugged.

The upshot of this is that in any large organisation you will spend a lot of time being called out to assist on these sore of issues.

I would also worry that as the PKI is an important security system you do really need to understand the theory and basics in order to make it or f value and not compromise it.

There are a lot of complex descriptions but Shon Harris does it well in her CISSP book or for a more in depth introduction Fundamentals of Cryptography by Bruce Schneier is a good place

Perhaps you could give us some information on the PKI, entrust, microsoft etc and the size of your organisation?

A CA really doesn't need someone to baby-sit it full time. Of course, good docs and such are important, but they're just not complicated enough to hire a Guru. If you don't have any real idea about what you're doing, hiring someone who Really Knows What Their Doing might be warranted to get it setup, but I can't imagine it'd take more than a couple of days of time to really get it sorted out, regardless of what you're using it for.