Hi Leah
Half an hour to spare!! Wow! I was always running late

Every investigation is dif.... er... I mean, I don't know what kind of interview you were doing, but in my experience doing liability and claims factuals for donkey's years, I usually took 2 to 2.5 hours to interview a claimant/plaintiff/insured. The other interviews relating to a specific case are typically witnesses, manager/supervisor and employer/CEO and they varied between short half-hour interviews up to several hours where the employer had to cover a myriad areas including detailed responses to a statement of claim.
Mind you, I must add that I always took statements. My clients expected statements and decs instead of audio, so my examples might not mean much to you if you are going to do audio.
Therefore I am only able to guess how you might assess your skills from the audio when you listen to it later. I know that on occasions when I was on video or a tape for whatever personal/family reasons, I'd often cringe at how I said something, or how stupid I was in making a certain comment. So I guess if you listen to your audio of the interview and you cringe every now and then, it will be a good guide for you.

Feel free to share (broadly) what kind of matter you were dealing with.
BgiGeorge