Even if you weren't going to get fired for not showing up, it's certainly unfair to the client.  You've got a professional obligation, and you should be there on time. 

Now, what's this got to do with you becoming a CCNA or CCNP?  Plenty.  Because when it comes to your study time, you're the client.  You owe it to yourself to show up.  You would never blow off an appointment to meet a client to get some important work done. 

First, though, you have to make that appointment with yourself!  Schedule your CCNA / CCNP study time, and keep that appointment as you would with a client.  Turn off the TV, your cell, your iPod, and everything else electronic that you carry around.  Believe it or not, the world can survive with being in contact with you for an hour or so!  You might even like it!

Getting certified isn't about how many hours, days, or weeks you spend studying.  It's about how much quality time you put in.  Be honest with yourself and realize that you're better off with 45 minutes of uninterrupted study as you would be with three hours of constantly interrupted study.