Time signatures... most songs have what's known as a 4/4 pulse. You can count "one, two, three, four" and the pattern will repeat again. If not 4/4, then certainly 3/4 (i.e., "one two three one two three"). It's a simple time signature - easy to dance to, easy to drum to, uses familiar structures, etc.
Listen to a song like Panic Attack, though, and while you can count "one, two, three, four" at first, by the time it gets to the verse you'll find the patterns repeat every five beats. Or, in a more complex song - such as the midsection to Sacrificed Sons - 10, then 9, then 10 again...
Just try and count along with the beats. The melodies don't arrive at regular intervals. It's very much a "feeling" thing - if you wanted, you could argue that every song ever was in 1/4, and start counting "one, one, one," but if you can keep a steady count and recognise where the riffs end, you'll get there.