Stating the rules
This will take about half an hour.
I'll take about ... minutes of your time.
I intend/I'm going to talk to you for about ... minutes.
Please stop me if you don't understand.
If you have any questions, please feel free to interrupt.
Feel free to ask questions during the presentation.
To save time/In the interests of time, could we leave questions until the end.
If you have any questions, I'll be glad to try and answer them at the end of my talk.
