Note Lengths

Hi,
This sites killa, great lessons and loads of support in the forum from what I see. Well done everyone!

Lets use Gabriel's 'Rocker Riff' because I'm familiar with it.
4 beats to the bar and I understand about different note lengths.
I can count it when the notes are all the same length.
The first 4 bars i can play because they have a rhythm I can follow, but i don't know how to count it.
I tap my foot on the beat but what about the lengths of the notes for that beat, if they are different what then?
After the 4th bar I get stuck because it doesn't have a rhythm I can follow. Is there a way to count the time over a beat if it has 8th,16th notes or any combination of notes?
Once I know how to do this then everything will be so much easier.
I'm sure it's easy and even been asked before, but searching this site for an answer takes up playing time so I'll just ask it's easier.

Thanks for any help.