Can anyone describe what a riff is because I am a little unsure. Could anyone point it out in a classic rock song. And also how long can a riff be?
A riff is something played by the guitarist that is often repeated a few times in the song. An example of a riff the well known part of Smoke on the Water. And i suppose it can be any length, but i'm not sure.
Let me google that for you

1  /rɪf/ Show Spelled Pronunciation [rif] Show IPA Jazz.
Use riff in a Sentence
1. a melodic phrase, often constantly repeated, forming an accompaniment or part of an accompaniment for a soloist.
–verb (used without object)
2. to perform riffs.

Source: Dictionary.com