it depends on your experience and the type of video you are working on. normally, professional videos take around 5-10 hours of work, to produce a 5-minute video. if it is so professional like cinematic videos it can take several days to work on. if the video is just depending on editing and colour grading it can take to almost 5-6 hours.
hope you like my answer.
If you are self-employed, the question then becomes one of billing. I used to do legal support work for attorneys. I billed by the hour, but, in all honesty, I didn't bill for all the hours I actually put into it - just what I felt was reasonable. Sometimes I'd be working on something and hit a mental block - it wasn't right to bill for the time it took me to work through it!
If you are billing by the job, then, you need to figure out what you feel is reasonable, and competitive in your market. Don't under-value your worth!
Also, please start now developing a portfolio of your work product, someplace where you keep things you can show to perspective clients. Also develop references. Contact info for past clients, and, get their permission to use them as references.
Self-confidence is hard to develop. The better you are, the more you do, the more confidence you will develop.
Like Armani, a Marine, said above, take your time and make it perfect!