How to get video and audio length

I'm trying to build a script where it is able to read the current time on a video or an audio file.

for example. I have a video/audio file that is 4 minutes long

1. get total lenght of video/audio file
2. I want to be able to go back at different time intervals and go in the current video/audio file amd get time gone by

ex: If video/audio file is at time 2.30sec, need script to pull that time out

Any ideas.Thank you!