I had to do a presentation at the beginning of my viva and it lasted 20 minutes (standard conference presentation time).
First, I wasn't sure that I liked the idea of a presentation but actually what I found is that it helped me point out the important points (in my view) in my thesis. I didn't cover everything that was in the thesis. The presentation was in the end the basis for most of the questions I got (as well as others but on the whole that is what we talked about). My viva lasted 2 hours and 45 mins (on top of the 20 minute-presentation) and was successful!
I hope it all goes well and that you find it somehow enjoyable. My supervisor told me before I sat my viva that there aren't many opportunities to talk about your work (and remember at this point in time, you are the most expert person in your topic) and be listened for 3 hours. And he was right!
Good luck!