I was surprised to learn (or simply to hear as I'm not equipped to judge the accuracy of the statement) that medicine is a lot like this: doctors have to spend a lot of time learning new procedures as what they know is basically obsolete within 5-10 years, which surprised me.
In IT of course it's more like 2-3 years. Of course some things are universal, which is why a decent education in the theory is so important but the hype in 5 years will be about something that probably hasn't been invented yet.
I personally find it's more important to keep abreast of the trends in the industry rather than using and becoming competent (let alone expert) in all of them (or even a significant subset). Like Ruby on Rails, Hibernate, JPA and pretty much any other ORM are about the same philosophical desire to diminish or remove the object-relational mismatch. You don't need to learn all of those technologies to know that. Just 1-2 will do.
I find it incredibly useful to skim relevant sites, particularly aggregator sites like www.dzone.com but also sites like InfoQ. 95% of the articles I don't read more than the blurb and those that read I'll often skim large parts. The sum of all these articles gives you a good idea of what's going on in the industry over time without reading them being a full-time job.
Your job will dictate a lot of what you use and most jobs won't change that much over time, certainly not at the pace the industry changes. Java Web applications have well and truly moved on from Struts (1) but you'll find there's plenty of people out there maintaing Struts code for their jobs.
To a certain extent, keeping the same job for many years is an exercise in forced obsolescence.
It's one reason (I believe) that staff turnover in IT is in the order of 18-24 months.
Don't think that you have to know everything. Just be aware that something exists and with enough experience you'll be able to pick up anything if you need to... as long as you know it exists and (at a high level anyway) it's purpose, strengths, limitations, etc.
I can't stress this point enough:
This is not an industry where you work 9-5 and that's it.
This industry will often involve longer hours and learning on your own time. Not because they have to (even though they kinda do) but because they want to.
I go into this subject in a bit more detail in Help me sell Linq training to my boss. Frankly, the whole attitude that you only have to learn things on the job and it's your boss's responsibility to teach you something (or give you the time to learn it) irks me no end.