If it's a first meet, I will usually only do 30 min duration, although if she's well reviewed and I feel 45 mins is GVFM then I may take a chance on that length of booking...
If I'm seeing Regs I will mainly do 45 mins (my preferred duration) with maybe an odd hour thrown in every now and then, as I know what I'm getting and it's going to ,more than likely be a great booking

...Therefore, I don't feel 30 mins is long enough to "cover all the bases"...

I don't ultimately think duration reflects on service received though...